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Introduction to Aeronautical Engineering Series 

a Electrical 


Introduction to Aeronautical 
Engineering Series 




EHJ Pallett 


Pitman Publishing 

First published 1976 

Pitman Publishing Lid 

Pitman House, 39 Parker Street, London WC2B 5PB, UK 

Pitman Publishing Corporation 

6 Davis Drive, Belmont, California 94002, USA 

Pitman Publishing Pty Ltd 

Pitman House, 158 Bouverie Street, Carlton, Victoria 3053, Australia 

Pitman Publishing 

Copp Clark Publishing 

517 Wellington Street West, Toronto M5V 101, Canada 

Sir Isaac Pitman and Sons Ltd 

Banda Street, PO Box 46038, Nairobi, Kenya 

Pitman Publishing Co Sa (Pty) Ltd 

Craighall Mews, Jan Smuts Avenue, Craighall Park, 

Johannesburg 2001, South Africa 

fciS. <%hl 

Class no. 


P25 1 8 


3 NOV 1976 

UAI cbuKl' 

©F.HJPallett 1976 

All rights reserved. No part of this publication 
may be reproduced, stored in a retrieval system, 
or transmitted, in any form or by any means, 
electronic, mechanical, photocopying, recording 
and/or otherwise, without the prior written 
permission of the publishers. 

ISBN 273 36159 7 


Text set in 10/1 1 pt. IBM Press Roman, printed by 
photolithography, and bound in Great Britain at 
The Pitman Press, Bath. 



Increases in size and speed, changes in shape and 
functional requirements of aircraft have each been 
possible by technical research and development and 
the progress made not only applies to those visible 
structural parts, but also to those unseen systems and 
services which enable it to function as an integrated 

A system ranking very highly indeed in this pro- 
gression is the one concerned with electrical power 
involving as it does various methods of generation, 
distribution, control, protection and utilization. These 
methods do, in fact, form a natural "build-up" of an 
aircraft's electrical system and their sequence sets a 
convenient pattern on which a study of principles and 
applications can be based. The material for this book 
therefore follows this pattern. 

In the early days of what is familiarly called "air- 
craft electrics", there was a certain distrust of the 
equipment. Although there was acceptance of the fact 
that electricity was necessary for operating the "wire- 
less" equipment, a few lights and an engine ignition 
system, many individuals were inclined to the view 
that if other systems could not be operated either by 
air, hydraulic oil, cables, numerous mechanical link- 
ages or petrol, then they were quite unnecessary! A 
majority of the individuals were mechanics, and the 
ground engineers as they were then known, and un- 
doubtedly, when "electrickery" began proving itself 
as a system operating media, it came as a pleasant 
relief to leave all relevant work to that odd character, 
the electrician, who speaking in some strange jargon 
and by means of diagrams containing numerous mystic 
lines and symbols, seemed better able to cope with it 

With the continued development of the various 
types of aircraft, the sources of electrical power have 
also varied from the simple battery and wind-driven 
generator, through to the most complex multiple a.c. 

generating systems. Similarly, the application of power 
sources have varied and in conjunction with develop- 
ments in electronics, has spread into the areas of 
other systems to the extent of performing not only a 
controlling function but, as is now so often the case, 
the entire operating function of a system. As a result, 
the work of the electrician assumed greater importance 
and has become highly specialized, while other main- 
tenance specialists found, and continue to find it 
increasingly necessary to broaden their knowledge of 
the subject; indeed it is incumbent on them to do so 
in order to carry out their important duties. This also 
applies to pilots in order that they may meet the 
technical knowledge requirements appropriate to 
their duties and to the types of aircraft they fly. 

Fundamental electrical principles are described in 
many standard text books, and in preparing the 
material for this book it was in no way intended that 
it should supplant their educational role. However, it 
has been considered convenient to briefly review cer- 
tain relevant principles in the chapters on generation 
and conversion of power supplies, to "lead-in" to the 
subject and, it is hoped, to convey more clearly how 
they are applied to the systems described. In keeping 
with the introductory nature of the book, and perhaps 
more important, to keep within certain size limitations, 
it obviously has not been possible to cover all types 
of aircraft systems. However, in drawing comparisons 
it is found that applications do have quite a lot in 
common, and so the examples finally chosen may be 
considered sufficiently representative to provide a 
useful foundation for further specialized study. 

The details given embrace relevant sections of the 
various syllabuses established for the technical exam- 
ination of maintenance engineers and pilots by official 
organizations, training schools and professional soci- 
eties. In this connection, therefore, it is also hoped 
that the book will provide a useful source of reference. 


A selection of questions are given at the end of each 
chapter and the author is indebted to the Society of 
Licensed Aircraft Engineers and Technologists for 
permission to reproduce questions selected from 
examination papers. 

Valuable assistance has been given by a number of 
organizations in supplying technical data, and in 
granting permission to reproduce many of the illustra- 
tions, grateful acknowledgement is hereby made to 
the following - 
Amphenol Ltd. 
Auto Diesels Braby Ltd. 
Aviquipo of Britain Ltd. 
Belling & Lee Ltd. 

British Aircraft Corporation (Operating) Ltd. 
Britten-Norman Ltd. 
Cannon Electric (G.B.) Ltd. 

Dowty Electrics Ltd. 
Graviner (Colnbrook) Ltd. 

Hawker Siddeley Aviation Ltd. 

Honeywell Ltd. 

International Rectifier Co. (G.B.) Ltd. 

Lucas Aerospace Ltd. 

Newton Brothers (Derby) Ltd. 

Normalair-Garrett Ltd. 

Plessey Co., Ltd. 

SAFT (United Kingdom) Ltd. 

Sangamo Weston Ltd. 

Shell Aviation News. 

Smiths Industries Ltd. 

Standard Telephones & Cables Ltd. 

Thom Bendix. 

Varley Dry Accumulators Ltd. 

Finally, thanks are also due to the publishers for 
having patiently awaited the completion of sections of 
manuscript and also for having accepted a number of 
changes of subject. 





1. Power Supplies - D.C. Generators * 

2. Power Supplies - Batteries *& 

3. Power Supplies - Alternating Current 27 

4. Power Conversion Equipment ** 

5. Ground Power Supplies 65 

6. Measuring Instruments, Warning Indicators and Lights 71 

7. Power Distribution ** 

8. Circuit Controlling Devices ™ 

9. Circuit Protection Devices J*l 

10. Power Utilization - Components j~0 

1 1 . Power Utilization - Systems j ^° 
Index 156 


Power Supplies 



Energy for the operation of most electrically operated 
equipment in an aircraft is supplied by a generator 
which may be of the direct current (d.c.) or alterna- 
ting current (a.c.) type. In this chapter we arc con- 
cerned with generators serving as the source of pri- 
mary d.c. supply for an aircraft electrical installation 
and before going into constructional and operating 
details of some typical machines, a review of relevant 
fundamental current-generating principles will be 

Fundamental Principles 

A generator is a machine that converts mechanical 
energy into electrical energy by the process of electro- 
magnetic induction. In both d.c. and a.c. types of 
generator, the voltage induced is alternating; the 
major difference between them being in the method 
by which the electrical energy is collected and applied 
to the circuit externally connected to the generator. 
Figure 1.1(a) illustrates a generator in its simplest 
form, i.e. a single loop of wire " AB" arranged to 
rotate between the pole pieces of a magnet. The ends 
of the wire are brought together to form a circuit 
via slip-rings, brushes and the externally connected 
load. When the plane of the loop lies at right angles to 
the magnetic field (position 1, Fig. 1.1(b)) no voltage 
is induced in the loop. As the loop rotates through 
90 degrees the wires cut the lines of force at right 
angles until at position 2 the induced voltage is at a 
maximum. As the loop approaches the vertical position 
again the voltage decreases since the rate at which 
lines of force are cut diminishes. At position 3 the 
induced voltage is zero. If rotation is continued, the 
number of lines cut gradually increases, until at 270 
degrees (position 4) it is once again maximum, but 
as the cutting is in the opposite direction there is also 

a reversal of the direction of induced voltage. As 
rotation continues, the number of lines cut decreases 
and the induced voltage reduces to zero as the loop 
returns to position 1. Plotting of the induced voltage 
throughout the full cycle produces the alternating or 
sine curve shown. 


Slip rings 




Fig 1.1 

(a) Simple form of generator 

(b) Induced voltage 

To convert the a.c. produced into unidirectional or 
d.c, it is necessary to replace the slip-rings by a 
collecting device referred to as a commutator. This is 
shown in Fig. 1.2 (a) and as will be noted it consists of 

two segments insulated from each other and connected 
to the ends of the loop. The brushes are set so that 
each segment moves out of contact with one brush 
and into contact with the other at the point where the 
loop passes through the positions at which induced 
voltage is minimum. In other words, a pulsating 
current increasing to maximum in one direction only 
is produced as shown by the curve in Fig. 1 .2(b). 



Fig 1.2 

Conversion of a.c. to d.c. 

(a) Use of commutator 

(b) Current wave-form 

In order to smooth out the pulsations and to pro- 
duce a more constant output, additional wire loops 
and commutator segments are provided. They are so 
interconnected and spaced about the axis of rotation, 
that several are always in a position of maximum 
action, and the pulsating output is reduced to a ripple 
as indicated in Fig. 1.3. 

Generator Classifications 

Generators are classified according to the method by 
which their magnetic circuits are energized, and the 
following three classes are normally recognized - 
(1) Permanent magnet generators. 


Resultant e m.f. between brushes 



Fig 1.3 

Effect on output using several coils 

(2) Separately-excited generators, in which electro- 
magnets are excited by current obtained from a 
separate source of d.c. 

(3) Self-excited generators, in which electro- 
magnets are excited by current produced by the 
machines themselves. These generators are fur- 
ther classified by the manner in which the fixed 
windings, i.e. the electromagnetic field and 
armature windings, arc interconnected. 

In aircraft d.c. power supply systems, self-excited 
shunt-wound generators are employed and the follow- 
ing details are therefore related only to this type. 

Fixed Winding Arrangement 

Figure 1.4 illustrates the arrangement of the fixed 
windings of a basic four-pole machine suitable for use 
as a self-excited generator. The fixed portion of the 
armature circuit consists of the four brushes, the links 
connecting together brushes of like polarity and the 





Fig 1.4 

Fixed winding arrangements 

cables connecting the linked brushes to the terminals 
indicated A and A 1 . The four field coils are of high 
resistance and connected in series to form the field 
winding. They are wound and connected in such a 
way that they produce alternate North and South 
polarities. The ends of the windings are brought out to 
the terminals indicated as Z and Z 1 . 

Generator Characteristics 

The characteristics of a generator refer to the relation- 
ship between voltage and the current flowing in the 
external circuit connected to a generator, i.e. the 
load current, and there are two which may be closely 
defined. These are: the external characteristic or 
relationship between terminal voltage and load 
current, and the internal characteristic or relationship 
between the actual electromagnetic force (e,m.f.) 
generated in the armature windings and load current. 
These relationships are generally shown in the form of 
graphs, with the graph drawn for one particular speed 
of the generator. 

Self -excited Shunt-wound Generators 

Shunt-wound generators are one of three types in the 
self-excited class of machine and as already noted are 
used in aircraft d.c. power supply systems. The term 
"shunt-wound" is derived from the fact that the 
high-resistance field winding is connected across or 
in parallel with the armature as shown in Fig. 1.5. The 

Fig 1.5 

Connection of shunt-field winding 

armature current divides into two branches, one 
formed by the field winding, the other by the 
external circuit. Since the field winding is of high 
resistance, the advantage is gained of having maximum 
current flow through the external circuit and expendi- 
ture of unnecessary electrical energy within the gener- 
ator is avoided. 

Operating Principle and Characteristic 

When the armature is rotated the conductors cut the 
weak magnetic field which is due to residual magnet- 
ism in the electromagnet system. A small e.m.f. is 
induced in the armature winding and is applied to the 
field winding, causing current to flow through it and 
so increasing the magnetic flux. This, in turn, causes a 
progressive increase in the induced e.m.f. and field 
current until the induced e.m.f. and terminal voltage 
reach the steady open-circuit maximum. 

The characteristic for this type of generator is shown 
in Fig. 1 .6 and it will be observed that the terminal 

Generated em f 
Terminal voltage' 


\ \ 



I.Rtfop ,. - 

Load (omperes) 

Fig 1.6 

Characteristic of self-excited shunt-wound generator 

voltage tends to fall with increasing load current. This 
is due to the voltage drop (1R drop) in the armature 
winding and also to a weakening of the main flux by 
armature reaction. The fall in terminal voltage reduces 
the field current, the main flux is further weakened 
and therefore a further fall in terminal voltage is pro- 

If the process of increasing the load is continued 
after the full working load condition has been reached, 
the terminal voltage will fall at an increasing rate until 
it can no longer sustain the load current and they both 
fall to zero. With reduced excitation the external 
characteristic of a shunt-wound generator falls much 
more rapidly so that the point at which voltage collapse 
occurs will be reached with a much smaller load cur- 
rent. In practice, field current is adjusted to maintain 
constant voltage under all load conditions, by a voltage 
regulator the operation of which will be described later. 

Sometimes a generator will lose its residual magnet- 
ism or become incorrectly polarized because of heat, 
shock, or a momentary current in the wrong direction. 
This can be corrected by momentarily passing current 
through the field from the positive terminal to the 
negative terminal; a procedure known as "flashing the 

Generator Construction 

A typical self-excited shunt-wound four-pole genera- 
tor, which is employed in a current type of turbo-prop 
civil transport aircraft, is illustrated in Fig. 1 .7. It is 
designed to provide an output of 9 kilowatts at a con- 
tinuous current of 300 amperes (A) over the speed 
range of 4,500 to 8,500 rev/min. In its basic form the 
construction follows the pattern conventionally 
adopted and consists of five principal assemblies; 
namely, the yoke, armature, two end frames and 
brush-gear assembly. 


The yoke forms the main housing of the generator, 
and is designed to carry the electromagnet system 
made up of the four field windings and pole pieces. It 
also provides for the attachment of the end frame 
assemblies. The windings are pre-formed coils of the 
required ampere-turns, wound and connected in 
series in such a manner that when mounted on the 
pole pieces, the polarity of the field produced at the 
poles by the coil current is alternately North and 
South (see Fig. 1.4). The field windings are suitably 
insulated and are a close fit on the pole pieces which 
are bolted to the yoke. The faces of the pole pieces 
are subjected to varying magnetic fields caused by 
rotation of the armature, giving rise to induced e.m.f. 
which in turn produces eddy currents through the 
pole pieces causing local heating and power wastage. 
To minimize these effects the pole pieces are of 
laminated construction; the thin soft iron laminations 
being oxidized to insulate and to offer high electrical 
resistance to the induced e.m.f. 

During operation on load, the current flowing through 
the armature winding of a generator creates a magnetic 
field which is superimposed on the main field pro- 
duced by field-winding current. Since lines of force 
cannot intersect, the armature field distorts the main 
field by an amount which varies with the load; such 
distorting effect is termed armature reaction. If 

uncorrected, armature reaction produces two additional 
undesirable effects: (i) it causes a shift of the Magnetic 
Neutral Axis, i.e. the axis passing through two points 
at which no e.m.f. is induced in a coil, setting up re- 
active sparking at the commutator, and (ii) it weakens 
the main field causing a reduction in generated e.m.f. 
The position of the brushes can be altered to mini- 
mize these effects under varying load conditions, but 
a more effective method is to provide additional wind- 
ings in the electromagnet system, such windings being 
referred to as interpole and compensating windings. 

Interpole windings are wound on narrow-faced 
auxiliary pole pieces located midway between the 
main poles, and are connected in series with the 
armature. The windings are such that an interpole has 
the same polarity as the next main pole in the direction 
of rotation, and as the fluxes are opposite in direction 
to the armature flux, they can be equalized at all loads 
by having the requisite number of turns. 

In order to provide true correction of armature 
reaction, the effects produced by interpoles must be 
supplemented, since alone they cannot entirely elim- 
inate all distortion occurring at the main pole faces. 
Compensating windings are therefore connected in 
series with the interpole and armature windings, and 
located in slots cut in the faces of the main pole shoes. 
The sides of the coils thus lie parallel with the sides of 
the armature coils. The ampere-turns of the winding 
are equal to those of the armature winding, while the 
flux due to it is opposite in direction to the armature 


The effectiveness of interpoles in minimizing reactance 
sparking is limited by armature speed, and their applic- 
ation as individual components of a field-winding 
system is, therefore, restricted to generators operating 
over a narrow speed range, e.g. the designed range of 
the generator illustrated in Fig. 1.7. In the case of 
generators designed for operation over a wide range, 
e.g. 2850 rev/min up to 10,000 rev/min, the use of 
interpoles alone would produce a side effect resulting 
in reactance sparking as the generator speed is reduced 
from maximum to minimum. To counteract this, and 
for a given load on the generator, it is necessary to 
reduce the magnetomotive force (m.m.f.) of the 
interpoles. The desired effect may be obtained by 
winding auxiliary coils over the interpole coils 
and connecting them in series with the generator shunt 
field winding in such a way that each coil, when 
energized by shunt field circuit current, produces an 



_; o 





m.m.f. of opposite polarity to that produced by the 
interpole coil on the same pole shoe. An exact balance 
between reactance e.m.f. and commutation e.m.f. is 
maintained over the full working range of generator 
speed to assist in producing sparkless commutation. 

The armature assembly comprises the main shaft 
(which may be solid or hollow) core and main winding, 
commutator and bearings; the whole assembly being 
statically and dynamically balanced. In the generator 
shown, the shaft is hollow and internally splined to 
mate with splines of a drive shaft which passes through 
the entire length of the armature shaft. 

Armature windings are made up of a number of 
individual identical coils which fit into slots at the 
outer edges of steel laminations which form the core 
of the armature. The coils are made from copper 
strip and as security against displacement by centri- 
fugal force, steel wire (in some cases steel strip) is 
bound round the circumference of the armature. The 
ends of each coil are brought out to the commutator 
and silver brazed to separate segments, the finish of 
one coil being connected to the same segment as the 
beginning of another coil. The complete winding thus 
forms a closed circuit. The windings are invariably 
vacuum-impregnated with silicone varnish to main- 
tain insulation resistance under all conditions. 

In common with most aircraft generators, the 
commutator is of small diameter to minimize centri- 
fugal stressing, and is built up of long, narrow copper 
segments corresponding in number to that of the 
field coils (a typical figure is 5 1 coils). The segment 
surfaces arc swept by brushes which are narrow and 
mounted in pairs (usually four pairs) to maintain the 
brush contact area per segment — an essential pre- 
requisite for effective commutation. 

The armatures of all aircraft generators are 
supported in high efficiency ball or roller bearings, or 
in combinations of these two types. Where combina- 
tions are used in a single generator it will be found 
that the ball bearing is invariably fitted at the drive 
end of the armature shaft, and the roller bearing at 
the commutator end. This arrangement permits 
lateral expansion of the armature shaft, arising from 
temperature increases in the generator, without expos- 
ing the bearings to risk of damage. Bearings are lubrica- 
ted cither with a specified high-melting-point grease or 
lubricating oil and may be of the sealed or non- 
sealed types. Sealed grease-lubricated bearings are 
pre-packed by the manufacturer and require no further 

lubrication during the life of the bearing. Non-sealed 
grease-lubricated bearings are assembled with suffic- 
ient lubricant to last for the period of the generator 
servicing cycle. In general the lubricant for oil- 
lubricated bearings is introduced into the bearing 
through the medium of oil-impregnated felt pads. 
Seals are provided to prevent oil escaping into the 
interior of the generator. 

These assemblies are bolted one at each end of the 
yoke and house the armature shaft bearings. The drive 
end frame provides for the attachment of the genera- 
tor to the mounting pad of the engine or gear-box 
drive (see also p. 8) and the commutator and 
frame provides a mounting for the brush-gear assem- 
bly and, in the majority of cases, also provides for 
the attachment of a cooling air duct. Inspection and 
replacement of brushes is accomplished by removing 
a strap which normally covers apertures in the commu- 
tator end frame. 


The brush-gear assembly is comprised of the brushes 
and the holding equipment necessary for retaining the 
brushes in the correct position, and at the correct 
. angle with respect to the magnetic neutral axis. 

Brushes used in aircraft generators are of the electro- 
graphitic type made from artificial graphite. The 
graphite is produced by taking several forms of 
natural carbons, grinding them into fine powder, 
blending them together and consolidating the mixture 
into the desired solid shape by mechanical pressure 
followed by exposure to very high temperature in an 
electric furnace. These brushes possess both the 
robustness of carbon and the lubricating properties of 
graphite. In addition they are very resistant to burn- 
ing by sparking, they cause little commutator wear 
and their heat conductivity enables them to with- 
stand overloads. 

As stated earlier, an essential prerequisite for 
effective commutation is that brush contact area per 
commutator segment should be maintained. This is 
accomplished by mounting several pairs of brushes in 
brush holders; in the generator illustrated in Fig. 1 .7 
four pairs of brushes are employed. The holders take 
the form of open-ended boxes whose inside surfaces 
are machined to the size of a brush, plus a slight clear- 
ance enabling a brush to slide freely without tilting or 
rocking. Contact between brushes and commutator is 
maintained by the pressure exerted by the free ends 

of adjustable springs anchored to posts on the brush 
holders. Springs are adversely affected by current 
passing through them; it is usual, therefore, to fit an 
insulating pad or roller at the end of the spring where 
it bears on the top surface of the brush. 

The brush holders are secured either by bolting 
them to a support ring (usually called a brush rocker) 
which is, in turn, bolted to the commutator end frame, 
or as in the case of the generator illustrated, bolted 
directly to the end frame. In order to achieve the 
best possible commutation a support ring, or end 
frame, as appropriate, can be rotated through a few 
degrees to alter the position of the brushes relative to 
the magnetic neutral axis. Marks are provided on each 
generator to indicate the normal operating position. 

When four or more brush holders are provided, they 
are located diametrically opposite and their brushes 
are alternately positive and negative, those of similar 
polarity being connected together by bar and flexible 
wire type links. 

The brushes are fitted with short leads or "pigtails" 
of flexible copper braid moulded into the brush dur- 
ing manufacture. The free ends of the pigtails termin- 
ate in spade or plate type terminals which are con- 
nected to the appropriate main terminals of the genera- 
tor via the brush holders and connecting links. 


The leads from brush-gear assemblies and field windings 
are connected to terminal posts secured to a block 
mounted on the commutator end frame or, in some 
generators, on the yoke assembly (see Fig. 1.7). The 
terminals and block are enclosed in a box-like cover 
also secured to the end frame. Entry for the output 
supply cables of the distribution system (refer to 
Chapter 7) is through rubber clamps. The rotation of 
a generator armature is specified in a direction, norm- 
ally anti-clockwise, when viewed from the drive end 
assembly. A movable link is fitted between two of the 
terminals which can be connected in an alternative 
position should it be necessary for the generator to be 
driven in the reverse direction. 


Sparking at the brushes of a generator, no matter how 
slight, results in the propagation of electromagnetic 
waves which interfere with the reception of radio sig- 
nals. The interference originating in generators may be 
eliminated quite effectively by screening and suppres- 
sion. Screening involves the enclosure of a generator 
in a continuous metallic casing and the sheathing of 

output supply cables in continuous metallic tubing or 
conduit to prevent direct radiation. To prevent inter- 
ference being conducted along the distribution cable 
system, the screened output supply cables are termin- 
ated in filter or suppressor units. These units consist 
of chokes and capacitors of suitable electrical rating 
built into metal cases located as close to a generator 
as possible. Independent suppressor units are rather 
cumbersome and quite heavy, and it is therefore the 
practice in the design of current types of generator to 
incorporate internal suppression systems. These 
systems do not normally contain chokes, but consist 
simply of suitably rated capacitors (see Fig. 1.7) 
which are connected between generator casing (earth) 
and terminals. The use of internal suppression systems 
eliminates the necessity for screened output supply 
cables and conduits thereby making for a considerable 
saving in the overall weight of a generator installation. 


The carbon from which electro-graphitic brushes are 
made is extremely porous and some of the pores are 
so very fine that carbon has an exceptional ability to 
absorb other substances into its structure, and to 
retain them. Moisture is one of these substances and 
it has for long been known that atmospheric moisture 
plays an important part in the functioning of a brush 
contact by affording a substantial degree of lubrication. 
The moisture is trapped under the inevitable irregular- 
ities of the contact faces of the brushes and forms an 
outside film on the commutator and it is with this 
film that the brushes make contact. Just how vital a 
part moisture does play was, however, not fully realized 
until aircraft began operating at high altitudes and the 
problem arose of brushes wearing out very rapidly 
under these conditions. Investigations into the problem 
showed that the fundamental difficulty was the 
extreme dryness of the atmosphere, this, in its turn, 
producing three secondary effects: (i) friction between 
brushes and commutator because the lubricating film 
cannot form, (ii) contact resistance becomes negligible 
giving rise to heavy reactive sparking and accelerated 
brush erosion and (hi) static electrical charges due to 
friction, producing molecular breakdown of the 


These effects have been largely eliminated by using 
brushes which have a chemical additive as a means of 
replacing the function which atmospheric moisture 
plays in surface skin formation. Two distinct categor- 
ies are in general use: brushes of one category form a 
constant-resistance semi-lubricating film on the com- 

mutator, while those in the other category are, in 
effect, self-lubricating brushes which do not form a 

The composition of the film-forming brushes 
includes chemicals (e.g. barium fluoride) to build 
up progressively a constant-resistance semi-lubricating 
film on the commutator surfaces. Brushes of this 
category do not wear abnormally at altitudes up to 
60,000 feet providing that generators to which they 
are fitted have been previously "bench run" for some 
hours to allow the formation of the protective film. 
This film, once formed, is very dark in colour and may 
often give the impression of a dirty commutator. 

Brushes of the non-film-forming category contain a 
lubricating ingredient such as molybdenum disulphide 
which is often packed in cores running longitudinally 
through the brushes. Since the brush is self-lubricating 
it is unnecessary for generators fitted with this type 
to be run for hours prior to entering service. However, 
they do have the disadvantage of appreciably shorter 
life, due to somewhat more rapid wear, when com- 
pared with film-forming brushes. 

Generator/engine Coupling 

A generator armature is coupled to its prime mover, the 
aircraft engine, via a shaft driven through gearing which 
forms part of an accessories gear-box. The required 
ratio of the gearing depends on the rated output of the 
generator and load requirements of an aircraft's elec- 
trical system and therefore varies. 

Drive shafts, usually known as quill drives, arc 
metal shafts with serrations or splines (either male or 
female) at one or both ends. The serrations or splines 
mate with corresponding formations on the generator 
armature shaft to transmit the torque delivered by the 
driving gear. One of the requirements to be satisfied by 
a quill drive is that it must effectively interrupt trans- 
mission of the driving torque in the event that the 
generator armature seizes up. This is done by design- 
ing the drive shaft so that at one section its diameter 
is smaller than the remaining sections; thus providing 
a weak spot at which the shaft will shear under the 
effect of an excessive torque. 

Quill drives are usually short and rigid, but in some 
cases a long drive with one end mating with serrations 
formed deep in a hollow armature shaft may be speci- 
fied. This arrangement enables the drive to absorb 
much of the mechanical vibration which is otherwise 
passed to a generator from an accessories gear-box. 
The method of securing a generator to an engine or 

an accessories gear-box varies, but in general it is either 
one utilizing a mounting flange or one requiring a 
manacle ring. In the mounting flange method, the end 
frame at the drive end of a generator is usually 
extended to a larger diameter than the yoke, thus form- 
ing a projecting flange. Holes in the flange line up with 
and accept studs which are located in the mounting 
pad of the engine or gear-box, and the generator is 
finally secured by nuts, locking washers, etc. An alter- 
native form of flange mounting is based on a generator 
end frame having two diameters. The larger diameter 
is no greater than that of the yoke and abuts on the 
mounting pad while the reduced diameter provides a 
channel or "gutter", between the yoke and the larger 
diameter of the end frame, into which the mounting 
studs project. Another variation of this form of 
mounting is employed in the generator shown in 
Fig. 1.7. 

In the manacle-ring method of mounting the 
generator drive end frame has an extension with a 
recess in the mounting face of the driving unit. When 
the generator extension is fully engaged with the 
recess, a flange on the end frame abuts on a matching 
flange formed on the driving unit mounting face. The 
two flanges are then clamped together by a manacle 
ring which, after being placed over them, is firmly 
closed by a tensioning screw. A spigot arrangement is 
usually incorporated to provide location of the gener- 
ator to the drive unit, and to absorb torque reaction 
when the generator is operating. 

Cooling of Generators 

The maximum output of a generator, assuming no 
limit to input mechanical power, is largely determined 
by the ease with which heat (arising from hysteresis, 
thermal effect of current in windings, etc.) can be 
dissipated. With large-bulk generators of relatively low 
output the natural processes of heat radiation from the 
extensive surfaces of the machine carcase may well 
provide sufficient cooling, but such "natural" cooling 
is inadequate for the smaller high-output generators 
used for the supply of electrical power to aircraft, and 
must, therefore, be supplemented by forced cooling. 
The most commonly accepted method of cooling 
is that which utilizes the ram or blast effect resulting 
from either the slipstream of a propeller or the air- 
stream due to the aircraft's movement. A typical cool- 
ing system is shown in a basic form in Fig. 1 .8. The air 
is forced at high speed into an intake and is led through 
light-alloy ducts to a collector at the commutator end 

of the generator. The air discharges over the brush-gear 
and commutator to cool this natural area of high tem- 
perature, and then passes through the length of the 
machine to exhaust through apertures, surrounded by 
a perforated strap, at the drive end. In order to assist 
in ram-air cooling and also to provide some cooling 
when the aircraft is on the ground, many types of 
generator have a fan fitted at the drive end of the 
armature shaft. 

Air scoop 

Engine nocelle 

end of generator 

Fig 1.8 

Typical cooling system 


Voltage Regulation 

The efficient operation of aircraft electrical equipment 
requiring d.c. depends on the fundamental requirement 
that the generator voltage at the distribution busbar 
system be maintained constant under all conditions of 
load and at varying speeds, within the limits of a pre- 
scribed range. It is necessary, therefore, to provide a 
device that will regulate the output voltage of a gener- 
ator at the designed value and within a specified toler- 

There are a number of factors which, either separately 
or in combination, affect the output voltage of a d.c. 
generator, and of these the one which can most con- 
veniently be controlled is the field circuit current, 
which in its turn controls the flux density. This con- 
trol can be effected by incorporating a variable resistor 
in series with the field winding as shown in Fig. 1.9. 
Adjustments to this resistor would vary the resistance 
of the field winding, and the field current and output 
voltage would also vary and be brought to the required 
controlling value. The application of the resistor in the 
manner indicated is, however, limited since it is essen- 


Control of field circuit current 

tial to incorporate a regulating device which will auto- 
matically respond to changes of load and speed, and 
also, automatically make the necessary adjustments to 
the generator field cunent. Two of the regulation 
methods commonly adopted are the vibrating contact 
method and the one based on the pressure/resistance 
characteristics of carbon, namely, the carbon pile 
method. In a few cases, regulation may also be based 
on semi-conductor circuit principles but, as this method 
is more often associated with certain a.c. generating 
systems, its operation will be covered in Chapter 3. 

Vibratiug Contact Regulator Vibrating contact regu- 
lators are used in several types of small aircraft employ- 
ing comparatively low d.c. output generators and a 
typical circuit for the regulation of both voltage and 
current of a single generator system is shown in basic 
form in Fig. 1.10. Although the coil windings of each 
regulator are interconnected, the circuit arrangement 
is such that cither the voltage regulator only or the 
current regulator only can operate at any one time. A 
third unit, called a reverse current cut-out relay, also 
forms part of some types of regulator, and since it has 
a circuit protection function, a description of its con- 
struction and operation will be given in Chapter 9. 

Voltage Regulator This unit consists of two windings 
assembled on a common core. The shunt winding con- 
sists of many turns of fine gauge wire and is connected 
in series with the current regulator winding and in 
parallel with the generator. The series winding, on the 
other hand, consists of a few turns of heavy gauge wire, 
and is connected in series with the generator shunt- 
field winding when the contacts of both regulators are 
closed, i.e. under static condition of the generator 
system. The contact assembly is comprised of a fixed 
contact and a movable contact secured to a flexibly- 


To distribution 

Fig. 1.10 
Vibrating contact regulator 

hinged armature. Movement of the armature and, 
therefore, the point at which contact opening and 
closing takes place is controlled by a spring which is 
pre-adjusted to the required voltage setting. 

When the generator starts operating, the contacts 
of both regulators remain closed so that a positive 
supply can flow through the generator shunt-field 
winding to provide the necessary excitation for raising 
the generator output. At the same time current passes 
through the shunt winding of the voltage regulator 
and, in conjunction with the series winding, it 
increases the regulator's electromagnetic field. As soon 
as the generator output voltage reaches the pre-adjusted 
regulator setting, the electromagnetic field becomes 
strong enough to oppose the tension of the armature 
spring thereby opening the contacts. In this equilibrium 
position, the circuit to the series winding is opened 
causing its field to collapse. At the same time, the 
supply to the generator field winding passes through a 
resistance (R) which reduces the excitation current 
and, therefore, the generator output voltage. The 
reduced output in turn reduces the magnetic strength 
of the regulator shunt winding so that spring tension 
closes the contacts again to restore the generator out- 
put voltage to its regulated value and to cause the 
foregoing operating cycle to be repeated. The frequency 
of operation depends on the electrical load carried by 

the generator; a typical range is between 50 to 200 
times a second. 

In regulators designed for use with twin-generator 
systems, a third coil is also wound on the electro- 
magnet core for paralleling purposes (see p. 13) and 
is connected to separate paralleling relays. 

Current Regulator This unit limits generator current 
output in exactly the same way as the voltage regulator 
controls voltage output, i.e. by controlling generator 
field-excitation current. Its construction differs only 
by virtue of having a single winding of a few turns of 
heavy wire. 

When electrical load demands are heavy, the voltage 
output value of the generator may not increase suffic- 
iently to cause the voltage regulator to open its con- 
tacts. Consequently, the output will continue to 
increase until it reaches rated maximum current, this 
being the value for which the current regulator is set. 
At this setting, the current flowing through the regu- 
lator winding establishes a strong enough electro- 
magnetic field to attract the armature and so open the 
contacts. Thus, it is the current regulator which now 
inserts resistance R in the generator shunt-field circuit 
to reduce generator output. As soon as there is sufficient 
drop in output the field produced by the regulator 
winding is overcome by spring tension, the contacts 


close and the cycle again repeated at a frequency 
similar to that of the voltage regulator. 

Carbon Pile Regulator Carbon has a granular surface 
and the contact resistance between two carbon faces 
that are held together depends not only on the actual 
area of contact, but also on the pressure with which 
the two faces are held together. If, therefore, a number 
of carbon discs or washers are arranged in the form 
of a pile and connected in series with the shunt field 
of a generator (see Fig. 1.11) the field circuit resistance 
can be varied by increasing or decreasing the pressure 
applied to the ends of the pile and changes in genera- 
tor output voltage therefore counteracted. Since this 
method eliminates the use of vibrating contacts, it is 
applied to generators capable of high current output, 
and requiring higher field excitation current. The 
necessary variation of pile pressure or compression 
under varying conditions of generator speed and load, 
is made through the medium of an electromagnet and 
spring-controlled armature which operate in a similar 
manner to those of a vibrating contact regulator. 

Under static conditions of the generator system, 
the carbon pile is fully compressed and since there is 
no magnetic "pull" on the armature, the resistance in 
the generator shunt-field circuit is minimum and the 
air gap between the regulator armature and electro- 
magnet core is maximum. As the generator starts 
operating, the progressively increasing output voltage 
is applied to the regulator coil and the resulting field 
establishes an increasing "pull" on the armature. 
During the initial "run-up" stages, the combination of 
low voltage applied to the regulator coil, and the 
maximum air gap between armature and core, results 
in a very weak force of attraction being exerted on the 
armature. This force is far smaller than that of the 
spring control, hence the armature maintains its original 
position and continues to hold the carbon pile in the 
fully compressed condition; the shunt-field circuit 
resistance is thus maintained at minimum value during 
run-up to allow generator output voltage to build up 
as rapidly as possible. This condition continues un- 
altered until the voltage has risen to the regulated 
value, and at which equilibrium is established between 
magnetic force and spring-control force. The armature 
is free to move towards the electromagnet core if the 
force of magnetic attraction is increased as a result 
of any increase in generator speed within the effective 
speed range. In these circumstances pile compression is 
further reduced so thai there is more air space between 
discs to increase resistance and so check a rise in 

generator output voltage; it also increases the spring 
loading that holds the armature away from the core. 
Thus, a condition of equilibrium is re-established with 
the armature in some new position, but with the 
output voltage still at the required regulated value. 

Any reduction of generator speed, within the 
effective speed range, produces a reduction in genera- 
tor output voltage thus disturbing regulator armature 
equilibrium in such a manner that the spring-control 
force predominates and the armature moves away 
from the electromagnet core. The carbon pile is re- 
compressed by this movement to reduce the generator 
shunt-field circuit resistance and thereby increase 
generator output voltage, until the regulated output 
is again brought to a state of equilibrium. When 
progressive reduction of generator speed results in a 
condition of maximum pile compression, control of 
generator output voltage is lost; any further reduction 
of generator speed, below the lower limit of the 
effective range, resulting in proportional decrease in 
output voltage. 

When a generator has been run up and connected 
to its distribution busbar system, the switching on of 
various requisite consumer services, will impose loads 
which disturb the equilibrium of the regulator 
armature. The effect is, in fact, the same as if the 
generator speed had been reduced, and the regulator 
automatically takes the appropriate corrective action 
until the output voltage is stabilized at the critical 
value. Conversely, a perceptible decrease in load, 
assuming generator speed to be constant and the 
regulator armature to be in equilibrium, results in the 
regulator taking the same action as in the case of an 
increase in generator speed. 

Construction The pile unit is housed within a ceramic 
tube which, in turn, is enclosed in a solid casing, or 
more generally, a finned casing for dissipating the 
heat generated by the pile. The number, diameter, and 
thickness, of the washers which make up the 
pile, varies according to the specific role of the 
regulator. Contact at each end of the pile is made by 
carbon inserts, or in some types of regulator by silver 
contacts within carbon inserts. The initial pressure of 
the pile is set by a compression screw acting through 
the pile on the armature and plate-type control spring 
which is supported on a bi-metal washer. The washer 
compensates for temperature effects on voltage coil 
resistance and on any expansion characteristics of the 
regulator, thus maintaining constant pile compression. 
The electromagnet assembly comprises a cylindrical 


00 3 




- tndjno 


yoke in which is housed the voltage coil, a detachable 
end-plate and an adjustable soft-iron core. A locking 
device, usually in the form of screws, is provided to 
retain the core in a pre-set position. 

Depending on the design of generating system, 
voltage regulators may be of the single-unit type, 
shown in Fig. 1.12, which operates in conjunction with 
separate reverse current cut-outs, voltage differential 
sensing relays and paralleling relays, or integrated with 
these components to form special control units or 

Fig 1.12 

Typical single unit type regulator 

1. Armature stop screw 

2. Magnet case 

3. Heat dissipator 

4. Terminal blocks 

5. Chassis 

Load-Sharing or Paralleling 

In multi-generator systems, it is necessary for the 
generators to operate in parallel, and in order to 
ensure that they carry equal shares of the system load, 
their output voltages must be as near equal as possible 
under all operating conditions. As we have already 

learned, generators are provided with a voltage regulator 
which exercises independent control over voltage out- 
put, but as variations in output and electrical loads 
can'occur, it is essential to provide additional voltage 
regulation circuits having the function of maintaining 
balanced outputs and load sharing. The method most 
commonly adopted for this purpose is that which 
employs a "load-equalizing circuit" to control gen- 
erator output via the voltage regulators. The principle 
as applied to a twin-generator system is illustrated in 
much-simplified form by Fig. 1.13(a). The generators 
are interconnected on their negative sides, via a series 
"load-sharing" or "equalizing" loop containing 
equalizing coils (C e ) each coil forming part of the 
individual voltage regulator electromagnetic circuits. 
The resistances R, and R 2 represent the resistances 
of the negative sections (interpole windings) of the 
generators, and under balanced load-sharing conditions 
the volts drop across each section will be the same, 
i.e. V, = I, R, and V 2 = I 2 R 2 .Thus, the net volts 
drop will be zero and so no current will flow through 
the equalizing coils. 

Let us now assume that generator No. 1 tends to 
take a somewhat larger share of the total load than 
generator No. 2. In this condition the volts drop V, 
will now be greater than V 2 and so the negative 
section of generator No. 1 will be at a lower potential. 
As a result, a current I c will flow through the equaliz- 
ing coils which are connected in such a manner that 
the effect of I e is to raise the output voltage of genera- 
tor No. 2 and reduce that of No. 1, thereby effect- 
ively reducing the unbalance in load sharing. 
Fig. 1.13(b) illustrates the principle as applied to an 
equalizing circuit which approximates to that of a 
practical generating system utilizing carbon pile volt- 
age regulators. The equalizing coils are wound on the 
same magnetic cores as the voltage coils of the regula- 
tors, thus, assuming the same unbalanced conditions 
as before, the current l e flows in a direction opposite 
to that flowing through the No. 2 generator voltage 
regulator coil, but in the same direction as the voltage 
coil current in No. 1 regulator. The magnetic effect of 
the No. 2 regulator voltage coil will therefore be 
weakened resulting in a decrease in carbon pile resis- 
tance and an increase in the output of No. 2 generator 
(see also p. 1 1), enabling it to take more of the load. 
The magnetic effect of the No. 1 regulator voltage coil 
on the other hand, is strengthened, thereby increasing 
carbon pile resistance and causing No. 1 generator to 
decrease its output and to shed some of its load. The 
variations in output of each generator continues until 


I, R,> V, 

V a % R 2 lj 



voltoge coil current 
Equalizing current I, 


Fig 1.13 

Principle of load-sharing 

the balanced load-sharing condition is once again 
restored, whereby the equalizing-circuit loop ceases 
to carry current. 

Test Questions 

1 . Describe how direct current is produced by a 

2. Describe how generators are classified, naming the 
three classes normally recognized and the class 

employed in aircraft systems. 

3. (a) Briefly describe armature reaction and the 

effects it has on generator operation, 
(b) How is armature reaction corrected in aircraft 

4. What is meant by reactance sparking? Explain how 
it is counteracted. 

5. In connection with generator brushes, state: 

(a) the materials from which they are made; 

(b) why several pairs of brushes are used. 


6. Briefly describe the causes of brush wear under 
high altitude flight conditions and the methods 
adopted for reducing wear. 

7. Which of the factors affecting the output voltage 
of a generator is normally controlled? 

8. With the aid of a circuit diagram, describe the 
fundamental principle of the carbon pile method 
of voltage regulation. 

9. Describe the operation of a vibrating contact type 
of voltage regulator. 

10. What additions must be made to voltage regulation 
circuits of a multi- generator system? 

1 1 . What is meant by "flashing the field" of a genera- 

12. What types of bearings are normally used in air- 
craft generators? 

13. With the aid of a circuit diagram describe how 
parallel operation of generators can be obtained. 

14. Describe means for cooling aircraft generators. 


Power Supplies— Batteries 

In almost all aircraft electrical systems a battery has 
the following principal functions - 

(i) To help maintain the d.c. system voltage under 
transient conditions. The starting of large d.c. motor- 
driven accessories, such as inverters and pumps, requires 
high input current which would lower the busbar volt- 
age momentarily unless the battery was available to 
assume a share of the load. A similar condition exists 
should a short circuit develop in a circuit protected by 
a heavy duty circuit breaker or current limiter. This 
function possibly applies to a lesser degree on aircraft 
where the electrical system is predominantly a.c, but 
the basic principle still holds true. 

(ii) To supply power for short term heavy loads 
when generator or ground power is not available, 
e.g. internal starting of an engine. 

(iii) Under emergency conditions, a battery is 
intended to supply limited amounts of power. UndeT 
these conditions the battery could be the sole remain- 
ing source of power to operate essential flight instru- 
ments, radio communication equipment, etc., for as 
long as the capacity of the battery allows. 

A battery is a device which converts chemical 
energy into electrical energy and is made up of a num- 
ber of cells which, depending on battery utilization, 
may be of the primary type or secondary type. Both 
types of cell operate on the same fundamental principle, 
i.e. the exchange of electrons due to the chemical 
action of an electrolyte and electrode materials. The 
essential differences between the two lies in the action 
that occurs during discharge. In the primary cell this 
action destroys the active materials of the cell, thus 
limiting its effective life to a single discharge operation, 
whereas in the secondary cell the discharge action con- 
verts the active material into other forms, from which 
they can subsequently be electrically reconverted, 
into the original materials. Thus, a secondary cell can 
have a life of numerous discharge actions, followed by 

the action of re-conversion more commonly known 
as charging. The batteries selected for use in aircraft 
therefore employ secondary cells and are either of the 
lead-acid or nickel-cadmium type. 

Lead-Acid Secondary Cell 

The basic construction of a typical cell is shown in 
Fig. 2.1. It consists essentially of a positive electrode 
and a negative electrode, each of which is, in turn, 
made up of a group of lead-antimony alloy grid plates; 
the spaces of the plates are packed with pastes of 
active lead materials. The two plate groups are inter- 
leaved so that both sides of every positive plate face a 
negative surface. The plates are prevented from com- 
ing into contact with one another by means of 
separators (not shown) made from materials having 

Vent cop 

Single battery cell 

showing positive and 

negative plate groups 

interlock eO. Plate 

separators are notshown 

Positive plate 

Fig 2.1 

Typical lead-acid secondary cell 


high insulating qualities and ability to permit un- 
obstructed circulation of the electrolyte at the plate 
surfaces. Each group of positive plates and negative 
plates is connected through a strap to a terminal post 
at the top and on opposite sides of the cell. Tire inter- 
nal resistance of a cell varies immensely with the dis- 
tance between the positive and negative electrode sur- 
faces; therefore, to obtain the lowest possible resistance 
the gap between the plates of each group is made as 
small as is practicable. A cell contains an odd number 
of plates, the outermost ones belonging to the negative 
plate group. The reason for this arrangement is that 
unlike a positive plate a negative plate will not distort 
when the electromechanical action is restricted to one 
side only. The plate assemblies of a cell are supported 
in an acid-proof container. 


Each positive plate of a fully-charged cell consists of 
the lead-antimony alloy grid into which lead peroxide 
paste (Pb0 2 ) has been forced under pressure. The 
negative plates are of similar basic structure, but with 
pure spongy lead (Pb) forced into the grid. The electro- 
lyte consists of two constituents, sulphuric acid 
(H 2 S0 4 ) and water, which are mixed in such prop- 
ortions that the relative density is generally about 
1-25 to 1-27. 

During discharge of the cell, that is, when an 
external circuit is completed between the positive and 
negative plates, electrons are transferred through the 
circuit from lead to lead peroxide and the net result 
of the chemical reaction is that lead sulphate (PbS0 4 ) 
forms on both plates. At the same time molecules of 
water are formed, thus weakening the electrolyte. For 
all practical purposes, the cell is considered to be dis- 
charged when both plates are covered with lead sul- 
phate and the electrolyte has become quite weak. 

The cell may be recharged by connecting the 
positive and negative plates, respectively, to the 
positive and negative terminals of a d.c. source of 
slightly higher voltage than the cell. All the fore- 
going reactions are then reversed; the lead sulphate on 
the positive plate being restored to lead peroxide, the 
negative plate restored to spongy lead, and the electro- 
lyte restored to its original relative density. 

Two types of lead-acid battery may be found in 
general use; in one the electrolyte is a free liquid while 
in the other it is completely absorbed into the plates 
and separators. An example of the former type of 

battery is illustrated in Fig. 2.2. The unit has a 24- 
volts output and consists of two 12-volt cell blocks 
moulded in high-impact plastic material and housed in 
an acid-proofed aluminium container. The links inter- 
connecting the cells and cell blocks are sealed and 
suitably insulated to prevent contact with the container. 
A plastic tray is fitted on to the top edges of the con- 
tainer and is sealed around the cell vent plugs by 
rubber pads and plastic sealing rings. The tray forms 
the base of a chamber for the ventilation of acid 
vapours. A plastic lid combined with an acid-proofed 
aluminium alloy hold-down frame completely 
encloses the chamber. Connections are provided at 
each end of the chamber for coupling the pipes from 
the aircraft's battery compartment ventilation system 
(see p. 22). 

Vent plug ondwosher 

'. .Plastic sealing ring 


Terminal snield 

Cell block 

Base mot Ooia plate 

[nsulo ting sneer 

ReO' pocumg 

Fig 2.2 

Lead-acid battery (free liquid type) 


Vent stopper 
and washer 

Connector bar 


Fig 2.3 

Lead-acid battery (absorbed liquid type) 

The battery illustrated in Fig. 2.3 utilizes a more 
specialized form of cell construction than that just 
described. The plates, active materials and separators 
are assembled together and are compressed to form a 
solid block. The active material is an infusorial earth, 
known as Kiesel Guhr, and is very porous and absor- 
bent. Thus, when the electrolyte is added, instead of 
remaining free as in the conventional types of battery, 
it is completely absorbed by the active material. This 
has a number of advantages; notably improved electro- 
mechanical activity, no disintegration or shedding of 
active material, thus preventing internal short-circuits 
caused by "sludge", low internal resistance and a 
higher capacity/weight ratio than a conventional 
battery of comparable capacity. 

The cells are assembled as two 1 2-volt units in 
monobloc containers made of shock-resistant poly- 
styrene and these are, in turn, housed in a polyester- 
bonded fibreglass outer container which also supports 
the main terminal box. A cover of the same material 
as the case is secured by four bolts on the end flanges 
of the case. 

Nickel-Cadmium Secondary Cell 

In this type of cell the positive plates are composed of 
nickel hydroxide, Ni(OH) 2 , the negative plates of 
cadmium hydroxide Cd(OH)i and the electrolyte is a 

solution of distilled water and potassium hydroxide 
(KOH) with a relative density of from 1 -24 to 1 -30. 
Batteries made up of these cells have a number of 
advantages over the lead-acid type, the most notable 
being their ability to maintain a relatively steady volt- 
age when being discharged at high currents such as 
during engine starting. 

The plates are generally made up by a sintering 
process and the active materials are impregnated into 
the plates by chemical deposition. This type of con- 
struction allows the maximum amount of active 
material to be employed in the electrochemical action. 
After impregnation with the active materials, the 
plates are stamped out to the requisite size and are 
built up into positive and negative plate groups, inter- 
leaved and connected to terminal posts in a manner 
somewhat similar to the lead-acid type of cell. 
Insulation is done by means of a fabric-base separator 
in the form of a continuous strip wound between the 
plates. The complete plate group is mounted in a 
sealed plastic container. 


During charging, the negative plates lose oxygen and 
become metallic cadmium. The positive plates are 
brought to a higher state of oxidation by the charging 
current until both materials are completely converted; 
i.e. all the oxygen is driven out of the negative plates 


and only cadmium remains, the positive plates pick 
up the oxygen to form nickel oxides. The cell emits 
gas towards the end of the charging process, and during 
overcharging; the gas being caused by decomposition of 
the water component of the electrolyte into hydrogen 
at the negative plates and oxygen at the positive plates. 
A slight amount of gassing is necessary to completely 
charge the cell and so it therefore loses a certain 
amount of water. 

The reverse chemical action takes place during dis- 
charging, the negative plates gradually gaining back 
the oxygen as the positive plates lose it. Due to this 
interchange there is no gassing on a normal discharge. 
In this way, the chemical energy of the plates is con- 
verted into electrical energy, and the electrolyte is 
absorbed by the plates to a point where it is not visible 
from the top of the cell. The electrolyte does not play 

an active part in the chemical reaction; it is used only 
to provide a path for current flow. 

The chemical reaction of a nickel-cadmium cell is 
summarized in Table 2.1 and may be compared with 
that taking place in a lead-acid battery cell. 

The construction of a typical battery currently in use 
is shown in Fig. 2.4. All the cells are linked and con- 
tained as a rigid assembly in the case. A space above 
the cells provides a ventilation chamber which is 
completely enclosed by a lid held in position by a 
pair of bolts anchored to the aircraft battery com- 
partment. Acid vapours are drawn out from the 
chamber via the vents in the battery case and the 
interconnecting pipes of the aircraft's battery com- 
partment ventilation system. 

Carrying handle 

Cell terminals 

Main battery 


Fig 2.4 

Nickel-cadmium type battery 


Battery Type 

State of 



Table 2.1 

Chemical Reactions of Batteries 

Positive Plate 

Negative Plate 

(Lead Dioxide) 

(Lead Sulphate) 

Ni,0, and Ni,0, 
(Nickel Oxides) 

(Nickel Hydroxide) 




(Lead Sulphate) 



(Cadmium Hydroxide) 

11, SO. 
Concentrated Sulphuric Acid 

H,S0 4 

Weak Sulphuric Acid 

KOH (Potassium hydroxide) 
unaffected by state of charge 

Capacity of Batteries 

The capacity of a battery, or the total amount of 
energy available, depends upon the size and number of 
plates. More strictly it is related to the amount of 
material available for chemical action. 

The capacity rating is measured in ampere-hours 
and is based on the maximum current, in amps, which 
it will deliver for a known time period, until it is dis- 
charged to a permissible minimum voltage of each 
cell. The time taken to discharge is called the discharge 
rate and the rated capacity of the battery is the pro- 
duct of this rate and the duration of discharge (in 
hours). Thus, a battery which discharges 7 A for 5 
hours is rated at 35 ampere-hours capacity. Some 
typical discharge rates of lead-acid and nickel- 
cadmium batteries are shown in Fig. 2.5. 

8 12 16 


Fig 2 JS 

Typical discharge rates of lead-acid and nickel-cadmium 



All batteries display certain indications of their state 
of charge, and these are of practical help in maintain- 
ing operating conditions. 

When a lead-acid battery is in the fully-charged 
condition each cell displays three distinct indications: 
the terminal voltage reaches its maximum value and 
remains steady; the relative density of the electrolyte 
ceases to rise and remains constant; the plates gas 
freely. The relative density is the sole reliable guide to 
the electrical condition of the cell of a battery which 
is neither fully charged nor yet completely discharged. 
If the relative density is midway between the normal 
maximum and minimum values then a cell is approxi- 
mately half discharged. 

Checks on the relative density of batteries which 
do not contain free electrolyte cannot be made; the 
state of charge being assessed only from voltage 

As we have already learned (see p. 19), the 
electrolyte in the cells of a nickel-cadmium battery 
does not chemically react with the plates as the 
electrolyte does in a lead-acid battery. Consequently, 
the plates do not deteriorate, nor does the relative 
density of the electrolyte appreciably change. For this 
reason, it is not possible to determine the state of 
charge by checking the relative density. Neither can 
the charge be determined by a voltage test because of 
the inherent characteristic that the voltage remains 
constant over a major part of the discharge cycle. The 
only possible check that a battery is fully charged is 


the battery voltage when "on-charge"; additionally, 
the electrolyte should be at maximum level under 
these conditions. 

Formation of white crystals of potassium carbonate 
on a properly serviced nickel-cadmium battery 
installed in an aircraft may indicate that the battery 
is being overcharged. The crystals form as a result of 
the reaction of expelled electrolyte vapour with carbon 


Batteries are capable of performing to their rated 
capacities when the temperature conditions and charg- 
ing rates are within the values specified. In the event 
that these are exceeded "thermal runaway" can occur, a 
condition which causes violent gassing, boiling of the 
electrolyte and finally melting of the plates and casing, 
with consequent danger to the aircraft structure and 
jeopardy of the electrical system. 

Since batteries have low thermal capacity heat can 
be dissipated and this results in lowering of the effect- 
ive internal resistance. Thus, when associated with 
constant voltage charging, a battery will draw a higher 
charging current and thereby set up the "runaway" 
condition of ever-increasing charging currents and 

In some aircraft, particularly those employing 
nickel-cadmium batteries, temperature-sensing 
devices are located within the batteries to provide a 
warning of high battery temperatures and to prevent 
overcharging by disconnecting the batteries from 
the charging source at a predetermined temperature 
(see also p. 25). 

Depending on the size of aircraft and on the power 
requirements for the operation of essential services 
under emergency conditions, a single battery or 
several batteries may be provided. When several 
batteries are employed they are, most often, con- 
nected in parallel although in some types of aircraft a 
series connection is used, e.g. two 14-volt batteries in 
series, while in others a switching arrangement is 
incorporated for changing from one method of con- 
nection to the other. 

Batteries are installed in individual compartments 
specially designed and located to provide adequate 
heat dissipation, ventilation of gases and protection 
of airframe structure against corrosive elements. At 
the same time batteries should be located as near 
to the main and battery busbars as physically possible 
in order to avoid the use of long leads and consequent 
high resistance. Batteries are normally mounted on, 
and clamped to, a tray secured to the aircraft structure. 
The tray forms a catchment for any acid which may 
escape from the battery. Trays may be of any material 
which is acid-proof, non-absorbent and resistant to 
reasonable impacts. Many reinforced plastics are suit- 
able but metal trays are, on the whole, undesirable. 
Where metal trays are unavoidable they are treated 
with an anti-corrosive paint or, in some cases, sprayed 
or coated with p.v.c. The structure under and around 
the battery area is also treated to avoid corrosive 
attack by acid fumes and spray. Batteries are securely 
clamped and anchored to their structure to prevent 
their being torn loose in the event of a crash landing, 
thus minimizing the risk of fire. Two typical battery 
installations are illustrated in Fig. 2.6. 

Mounting troy 

External supply 
Battery isololoi 



Bottery clomp 

'Pip' pin 

. Ventilation oir 


Battery venting 
pipe outlet 

Fig 2.6 

Typical battery installations 


Venting of batteries and battery compartments may 
take various forms since it depends largely on the 
installation required for a particular type of aircraft. 
Rubber or other non-corrosive pipes are usually 
employed as vent lines which terminate at ports in the 
fuselage skin so that the airflow over it draws air 
through the pipes by a venturi action. In some cases, 
acid traps, in the form of polythene bottles, are 
inserted in the lines to prevent acid spray being ejected 
on to the outer-skin of the aircraft. 

In the installation shown in Fig. 2.6(b) fumes and 
gases generated by the battery are extracted by the 
difference of pressure existing across the aircraft. 
During normal flight air tapped from the cabin 
pressurization system enters the battery ventilation 
chamber and continues through to the outside of the 
aircraft. On the ground, when no pressure differential 
exists, a non-return valve fitted in the air inlet prevents 
fumes and gases from escaping into the aircraft. These 
typical venting arrangements are illustrated schematic- 
ally in Fig. 2.7. 


f*ni — 

Acid trap 

t Pressunration 
' oir 

Non -return 

Fig 2.7 
Battery venting arrangements 


The method of connecting batteries to their respective 
busbars or power distribution points, depends largely 
on the type of battery employed, and on the aircraft's 
electrical system. In some cases, usually on the smaller 
types of aircraft, the connecting leads are provided 
with forked lugs which fit on to the appropriate 
battery terminals. However, the method most 
commonly employed is the plug and socket type 

connector shown in Fig. 2.8. It provides better con- 
nection and, furthermore, shields the battery terminals 
and cable terminations. 

Six safety wire 
holes 0.125 dio. 




Fig 2.8 

Battery plug connector 

The socket comprises a plastic housing, incorporated 
as an integral part of the battery, two shrouded plug 
pins and the female threaded portion of a quick-start 
thread lead-screw. The plug consists of a plastic 
housing incorporating two shrouded spring-loaded 
sockets and terminals for the connection of battery 
leads, and the male half of the mating lead-screw 
operated by a handwheel. The two halves, on being 
engaged, are pulled into position by the lead-screw 
which thereafter acts as a lock. Reverse rotation of 
the handwheel separates the connector smoothly with 
very little effort. In this way high contact pressures 
and low resistance connections are possible and are 
consistently maintained. 

Fig. 2.9 rhows the circuit arrangement for a battery 
system which is employed in a current type of 
turboprop airliner; the circuit serves as a general guide 
to the methods adopted. Four batteries, in parallel are 
directly connected to a battery busbar which, in the 
event of an emergency, supplies power for a limited 
period to essential consumer services, i.e. radio, fire- 

warning and extinguishing systems, a compass system, 
etc. Direct connections are made to ensure that 
battery power is available at the busbar at all times. 

The batteries also require to be connected to 
ensure that they are maintained in a charged condition. 
In the example illustrated this is accomplished by 
connecting the batteries to the main d.c. busbar via a 
battery relay, power selector switch and a reverse 
current circuit breaker. 

Under normal operating conditions of the d.c. 
supply system, the power selector switch is set to the 
"battery" position (in some aircraft this may be 
termed the "flight" position) and, as will be noted, 
current flows from the batteries through the coil of 

To generator systems 
and a II d.c. services • 

the battery relay, the switch, and then to ground via 
the reverse current circuit breaker contacts. The 
current flow through the relay coil energizes it, 
causing the contacts to close thereby connecting the 
batteries to the main busbar via the coil and second 
set of contacts of the reverse current circuit breaker. 
The d.c. services connected to the main busbar are 
supplied by the generators and so the batteries will 
also be supplied with charging current from this 

Under emergency conditions, e.g. a failure of the 
generator supply or main busbar occurs, the batteries 
must be isolated from the main busbar since their 
total capacity is not sufficient to keep all services in 

To essential 


Main bus 

Reverse current C/B 

Power selector 

switch &, 


Battery bus-bar 

Battery bus 

rj Voltmeter selector 

To external 

power circuit 

and ground 

power plug 

V Battery 


Battery relay 

— Current flow from batteries 

^_ Charging current flow from 




L • i 1 






J" J 

Fig 2.9 

Typical battery system circuit 




= 9- 


operation. The power selector switch must therefore 
be put to the "off position, thus de-energizing the 
battery relay. The batteries then supply the essential 
services for the time period pre-calculated, on the 
basis of battery capacity and current consumption of 
the essential services. 

The reverse current circuit breaker in the system 
shown is of the electromagnetic type and its purpose 
is to protect the batteries against heavy current flow 
from the main busbar. Should this happen the current 
reverses the magnetic field causing the normally closed 
contacts to be released; thus interrupting the circuit 
between the batteries and main busbar, and also to 
de-energize the battery relay. 

In several types of turbojet transport aircraft 
currently in service, the nickel-cadmium battery 
system incorporates a separate unit for maintaining 
the batteries in a state of charge, and in conjunction 
with temperature-sensing elements, the unit also 
automatically isolates the charging circuit whenever 
there is a tendency for battery overheating to occur. 
The circuits of such systems vary between aircraft 
types and a study of each is well beyond the limitations 
of this book. There are, however, similarities between 
applications of fundamental principles and these may 
be understood with the aid of Fig. 2.10. The circuit 
is based on the system adopted for the Douglas DC-10 
and is presented in much simplified form in order to 
illustrate how changeover to battery power takes 
place, and also the basic function of an "on-board" 
charging unit. 

In this particular application, the required output 
of 28 volts is achieved by connecting two 14-volt 
batteries in series. Furthermore, and unlike the system 
shown in Fig. 2.9, the batteries are only connected to 
the battery busbar whenever the normal d.c. supply 
(in this case from transformer/rectifier units) is not 
available. Connection to the busbar and to the 
charger unit is done automatically by means of a 
"charger/battery" relay and sensing relays. 

When power is available from the main generating 
system, d.c. is supplied to the battery busbar from a 
transformer/rectifier unit and, at the same time, to 
the coils of the sensing relays. With the relays ener- 
gized, the circuit through contacts A2-A3 is inter- 
rupted while the circuits through contacts B1-B2 are 
made. The battery switch, which controls the operation 
of the charger/battery relay, is closed to the "bat" 
position when the main electrical power is available, 
and the emergency power switch is closed in the "off 

The charger/battery relay is of the dual type, one 
relay being a.c. operated and the other d.c. operated. 
The a.c. relay coil is supplied with power from one 
phase of the main three-phase supply to the battery 
charger, and as will be noted from the diagram, the 
relay is energized by current passing to ground via the 
contacts B1-B2 of the sensing relays, the battery switch 
and the emergency switch. Energizing of the relay 
closes the upper set of contacts (A1-A2) to connect 
the d.c. positive output from the battery charger to 
the batteries, thereby supplying them with charging 

In the event of main power failure, the battery 
charger will become inoperative, the a.c. charger 
relay will de-energize to the centre off position, and 
the two sensing relays will also de-energize, thereby 
opening the contacts B1-B2 and closing the contacts 
A2-A3. The closing of contacts A2-A3 now permits a 
positive supply to flow direct from the battery to the 
coil of the d.c. battery relay, which on being energized 
also actuates the a.c. relay, thereby closing contacts 
Bl -B2 which connect the batteries direct to the battery 
busbar. The function of the battery relay contacts is 
to connect a supply from the battery busbar to the 
relays of an emergency warning light circuit. The charg- 
ing unit converts the main three-phase supply of 1 1 5/ 
200 volts a.c. into a controlled d.c. output at constant 
current and voltage, via a transformer and a full-wave 
rectifying bridge circuit made up of silicon rectifiers 
and silicon controlled rectifiers (see also p. 49). The 
charging current is limited to approximately 65 A, and 
in order to monitor this and the output voltage as a 
function of battery temperature and voltage, temper- 
ature-sensing elements within the batteries are 
connected to the S.C.R. "gates" via a temperature and 
reference voltage control circuit, and a logic circuit. 
Thus, any tendency for overcharging and overheating 
to occur is checked by such a value of gate circuit 
current as will cause the S.C.R. to switch off the 
charging current supply. 

Test Questions 

1. Describe the construction of a lead-acid battery and 
the chemical changes which occur during charging. 


2. Describe the construction of a nickel-cadmium 
battery and the chemical changes which occur 
during charging. 

3. The capacity of a battery is measured in: 
(a) volts. 


(b) cubic centimetres. gases from the battery compartment of an aircraft. 

(c) ampere-hours. 6. With the aid of a circuit diagram, describe a typical 

4. What indications would be displayed by a lead-acid method of maintaining batteries in a charged con- 
battery of the free electrolyte type, and a nickel- dition when they are installed in an aircraft 
cadmium battery, which would serve as a guide to 7. What do you understand by the term "thermal 
their state of charge? runaway"? 

5. Describe a typical method of extracting fumes and 


Power Supplies 


Before studying the operation of some typical genera- 
ting systems currently in use it will be of value to 
recapitulate certain of the fundamentals of alternating 
current behaviour, and of terminology commonly used. 


The voltage and current produced by the generator of 
an a.c. system build up from zero to a maximum of 
one polarity, then decay to zero, build up to a maxi- 
mum of opposite polarity, and again decay to zero. 
This sequence of build up and reversal follows a sine 
wave form and is called a cycle and the number of 
cycles in unit time (usually one second) is called the 
frequency (see Fig. 3.1). The unit of frequency 
measurement is the Hertz (Hz). 

Inston'oneous value 

itude or peak value 

Fig 3.1 

Cycle and frequency 

In a conventional generator, the frequency is 
dependent upon the speed of rotor rotation within its 
stator and the number of poles. Two poles of a rotor 
must pass a given point on the stator every cycle; 

Frequency (c.p.s.) = 

_ r.p.m. x pairs of poles 


For example, with a 6-pole generator operating at 
8000 r.p.m., 

Frequency = 


= 400 c.p.s. or 400 Hz 

For aircraft constant frequency systems (see p. 32) 
400 Hz has been adopted as the standard. 

At any given instant of time the actual value of an 
alternating quantity may be anything from zero to 
a maximum in either a positive or negative direction; 
such a value is called an Instantaneous Value. The 
Amplitude or Peak Value is the maximum instant- 
aneous value of an alternating quantity in the positive 
and negative directions. 

The wave form of an alternating e.m.f. induced in 
a single-turn coil, rotated at a constant velocity in a 
uniform magnetic field, is such that at any given 
point in the cycle the instantaneous value of e.m.f. 
bears a definite mathematical relationship to the 
amplitude value. Thus, when one side of the coil 
turns through 0° from the zero e.m.f. position and in 
the positive direction, the instantaneous value of e.m.f. 
is the product of the amplitude (£" max ) and the sine of 
6 or, in symbols: 

£inst ■ SmaxSin B 


The calculation of power, energy, etc., in an a.c. cir- 


cuit is not so perfectly straightforward as it is in a 
d.c. circuit because the values of current and voltage 
are changing throughout the cycle. For this reason, 
therefore, an arbitrary "effective" value is essential. 
This value is generally termed the Root Mean Square 
(r.m.s.) value (see Fig. 3.2). It is obtained by taking a 

Fig 3.2 

R.M.S. value of alternating current 

number of instantaneous values of voltage or current, 
whichever is required, during a half cycle, squaring 
the values and taking their mean value and then taking 
the square root. Thus, if six values of current "/" are 
taken, the mean square value is: 

/, 2 +/ 2 2 +/3 2 +y+/ 5 2 +/6 2 

and the r.m.s. value is: 


A 2 +/ 2 2 +/3 2 +/4 2 +/s 2 +/ 6 2 

The r.m.s. value of an alternating current is related 
to the amplitude or peak value according to the wave 
form of the current. For a sine wave the relationship 
is given by: 

r.m.s. = — g- = 0-707 Peak 

Peak =\/2 r.m.s. = 1-414 r.m.s. 

In connection with a.c. generating systems and 
associated circuits, the term "phase" is used to indicate 
the number of alternating currents being produced 
and/or carried simultaneously by the same circuit. 
Furthermore, it is used in designating the type of 
generating system and/or circuit, e.g. a "single- phase" 
system or one producing single-phase current, and a 
"polyphase" system or one producing several single 

alternating currents differing in phase. Aircraft poly- 
phase systems and circuits are normally three-phase, 
the three currents differing in phase from each other 
by 1 20 electrical degrees. 

The current and voltage in an a.c. circuit have the 
same frequency, and the wave form of the alternating 
quantities is similar, i.e. if the voltage is sinusoidal 
then the current is also sinusoidal. In some circuits the 
flow of current is affected solely by the applied volt- 
age so that both voltage and current pass through zero 
and attain their peaks in the same direction simul- 
taneously; under these conditions they are said to be 
"in phase". In many circuits, however, the current flow 
is influenced by magnetic and electrostatic effects set 
up in and around the circuit, and although at the same 
frequency, voltage and current do not pass through 
zero at the same instant. In these circumstances the 
voltage and current are said to be "out of phase", the 
difference between corresponding points on the wave- 
forms being known as the phase difference. The term 
"phase angle" is quite often used, and is synonymous 
with phase difference when expressed in angular 
measure. The phase relationships for the three basic 
forms of a.c. circuits, namely, pure resistive, induct- 
ive and capacitive, are illustrated in Fig. 3.3. 

In a pure resistive circuit (Fig. 3.3(a)) the resistance 
is constant, therefore magnetic and electrostatic effects 
are absent, and the applied voltage is the only factor 
affecting current flow. Thus, voltage and current are 
"in phase" in a resistive circuit. 

In a pure inductive circuit (normally some resistance 
is always present) voltage and current are always out- 
of-phase. This is due to the fact that a magnetic field 
surrounds the conductors, and since it too continually 
changes in magnitude and direction with the alternat- 
ing current, a self-induced or "reactance" e.m.f. is 
set up in the circuit, to oppose the change of current 
in the circuit. As a result the rise and fall of the 
current is delayed and as may be seen from Fig. 3.3(b) 
the current "lags" the voltage by 90 degrees. 

Capacitance in an a.c. circuit also opposes the 
current flow and causes a phase difference between 
applied voltage and current but, as may be noted from 
Fig. 3.3(c), the effect is the reverse to that of induct- 
ance, i.e. the current "leads" the voltage by 90 degrees. 

Where the applied voltage and current are out of 
phase by 90 degrees they are said to be in quadrature. 
A three-phase circuit is one in which three voltages 
are produced by a generator with three coils so spaced 
within the stator, that the three voltages generated 
are equal but reach their amplitude values at different 


times. For example, in each phase of a 400 Hz, three- 
phase generator, a cycle is generated every 1 /400 
second. In its rotation, a magnetic pole of the rotor 
passes one coil and generates a maximum voltage; one- 
third of a cycle (1/1200 second) later, this same pole 
passes another coil and generates a maximum voltage 
in it. Thus, the amplitude values generated in the three 
coils are always one- third of a cycle (120 electrical 
degrees; 1/1200 second) apart. 


Pure resistive — in phase 


Pure inductive — / lags behind E 


Pure copocitive — / leads E 

Fig 3.3 

A.C circuits phase relationship 

The interconnection of the coils to form the three 
phases of a basic generator, and the phase sequence, is 
shown in Fig. 3.4. The output terminals of generators 
are marked to show the phase sequence, and these 
terminals are connected to busbars which are identified 

°hase A 


Fig 3.4 

Three-phase system 

Each phase of a three-phase generator may be brought 
out to separate terminals and used to supply separate 
groups of consumer services. This, however, is an 
arrangement rarely encountered in practice since 
pairs of "line" wires would be required for each 
phase and would involve uneconomic use of cable. 
The phases are, therefore, interconnected normally 
by either of the two methods shown in Fig. 3.5. 

The "Star" connection ((a)) is commonly used in 
generators. One end of each phase winding is con- 
nected to a common point known as the neutral point, 
while the opposite ends of the windings are connected 
to three separate lines. Thus, two-phase windings are 
connected between each pair of lines. Since similar 
ends of the windings are joined, the two phase e.m.f.s 
are in opposition and out of phase and the voltage 
between lines (£"l) is the phase voltage (£" p h) multi- 
plied by \/3- For example, if £" pn is 120 volts, then 
£"l equals 120 x 1-732, or 208 volts approx. As far as 
line and phase currents are concerned, these are equal 
to each other in this type of circuit connection. 


If necessary, consumer services requiring only a 
single-phase supply can be tapped into a three-phase 
star-connected system with a choice of two different 
voltage levels. Thus, by connecting from one phase 
to neutral or ground, we obtain a single-phase 1 20 
volts supply while connecting across any pair of lines 
we can obtain a single-phase 208 volts supply. 

Neutral point 


I ■ ' Line vons = J% % F f 


£: = 208V 


VV3»/ P1 


Fig 3 .5 

Interconnection of phases 

(a) "Star" connection 

(b) "Delta" connection 

Figure 3.5(b) illustrates the "Delta" method of 
connection, the windings being connected in series to 
form a closed "mesh" and the lines being connected 
to the junction points. As only one phase winding is 
connected between each pair of lines then, in the 
delta method, line voltage (£"]_) is always equal to 
phase voltage (£"ph). The line current, however, is the 
difference between the phase currents connected to 
the line and is equal to the phase current (/ p h) 
multiplied by \/3- 

The power ratings of a.c. generators are generally 
given in kilovolt-amperes (kVA) rather than kilowatts 
(kW) as in the case of d.c. machines. The primary 
reason for this is due to the fact that in calculating 
the power, account must be taken of the difference 

between the true or effective power, and the apparent 
power. Such a difference arises from the type of 
circuit which the generator is to supply and the phase 
relationships of voltage and current, and is expressed 
as a ratio termed the power factor (P.F.). This may 
be written: 

_ Effective Power (kW) 
Apparent Power (kVA) 

= cosine phase angle \p 

If the voltage and current are in phase (as in a 
resistive circuit) the power factor is 1 00 per cent or 
unity, because the effective power and apparent power 
are equal; thus, a generator rated at 100 kVA in a cir- 
cuit with a P.F. of unity will have an output 100 per 
cent efficient and exactly equal to 100 kW. 

When a circuit contains inductance or capacitance, 
then as we have already seen (p. 28) current and 
voltage are not in phase so that the P.F. is less than 
unity. The vector diagram for a current / lagging a 
voltage E by an angle <p is shown in Fig. 3.6. The 
current is resolved into two components at right 
angles, one in phase with E and given by / cos f>, and 
the other in quadrature and given by / sin *p. The 
in-phase component is called the active, wattful or 
working component (kW) and the quadrature com- 
ponent is the idle, wattless or reactive component 


Reocitve component / sin tp 


Fig 3.6 

Components of current due to phase difference 


(WAR). The importance of these components will 
be more apparent when, later in this chapter, methods 
of load sharing between generators are discussed. 
Most a.c. generators are designed to take a pro- 
portion of the reactive component of current through 
their windings and some indication of this may be 
obtained from the information given on the generator 
data plate. For example, the output rating may be 
specified as 40 kVA at 0-8 P.F. This means that the 
maximum output in kW is 0-8 x 40 or 32 kW, but 
that the product of volts and amperes under all 
conditions of P.F. must not exceed 40 kVA. 

A frequency-wild system is one in which the frequency 
of its generator voltage output is permitted to vary 
with the rotational speed of the generator. Although 
such frequency variations are not suitable for the 
direct operation of all types of a.c. consumer equip- 
ment, the output can (after constant voltage regula- 
tion) be applied directly to resistive load circuits such 
as electrical de-icing systems, and can also be trans- 
formed and rectified (see Chapter 4) to provide medium- 
or low-voltage d.c. Several types of aircraft currently 
in service employ frequency-wild generators in either 
or both of the foregoing applications, and some 
details of the construction and operation of two 
representative machines are given in the following 

Generator Construction 

The construction of a typical generator utilized for 
the supply of heating current to a turbo-propeller 
engine de-icing system is illustrated in Fig. 3.7. It has 
a three-phase output of 22 kVA at 208 volts and it 
supplies full load at this voltage through a frequency 
range of 280 to 400 Hz. Below 280 Hz the field 
current is limited and the output relatively reduced. 
The generator consists of two major assemblies: a 
fixed stator assembly in which the current is induced, 
and a rotating assembly referred to as the rotor. The 
stator assembly is made up of high permeability lam- 
inations and is clamped in a main housing by an end 
frame having an integral flange for mounting the 
generator at the corresponding drive outlet of an 
engine-driven accessory gear-box. The stator winding 
is star connected, the star or neutral point being made 
by Unking three ends of the winding and connecting 
it to ground (see also p. 30). The other three ends 
of the winding are brought out to a three-way output 

terminal box mounted on the end frame of the 
generator. Three small current transformers are fitted 
into the terminal box and form part of a protection 
system known as a Merz-Price system. 

Outpul tetmmol 

End froroe 

Air oullet soout 

Fig 3.7 

Frequency-wild generator 

The rotor assembly has six salient poles of lamina- 
ted construction; their series-connected field windings 
terminate at two slip rings secured at one end of the 
rotor shaft. Three spring-loaded brushes are equispaced 
on each slip ring and are contained within a brush-gear 
housing which also forms a bearing support for the 
rotor. The brushes are electrically connected to d.c. 
input terminals housed in an excitation terminal box 
mounted above the brush-gear housing. The terminal 
box also houses capacitors which are connected 
between the terminals and frame to suppress inter- 
ference in the reception of radio signals. At the drive 
end, the rotor shaft is serrated and an oil seal, housed 
in a carrier plate bolted to the main housing, is fitted 
over the shaft to prevent the entry of oil from the 
driving source into the main housing. 

The generator is cooled by ram air (see also Chapter 
1 , p. 8) passing into the main housing via an inlet 
spout at the slip ring end, the air escaping from the 
main housing through ventilation slots at the drive- 
end. An air-collector ring encloses the slots and is 
connected to a vent through which the cooling air is 
finally discharged. Provision is made for the installa- 


tion of a thermally-operated switch to cater for an 
overheat warning requirement. 

A frequency-wild generator of a type employed in 
a variety of single-engined and twin-engined aircraft is 
shown in Fig. 3.8. It is designed to supply a primary 
d.c. system of the purely rectified type, and as will be 
particularly noted, it is driven by a pulley and belt 
system in the manner usually adopted for automobile 
generators. Its operating frequency is about 100 Hz 
at idling speed of the engine and increases with speed 
to 1200 Hz or higher. 


Seoinng bolt 

S*cunng bolt 

cooling pipe 

Fig 3.8 

Frequency-wild generator 

The basic construction of the generator follows 
the general pattern in that it consists of a rotor, stator, 
slip-ring and brush assembly and end frames. In addition 
six silicon diodes are carried in an end frame and are 
connected as a bridge rectifier (see p. 51) to provide 
the d.c. for the aircraft's system. The principal con- 
structional features are illustrated in Fig. 3.9. 

Fig 3.9 

Sectioned view frequency-wild generator 

The rotor is formed by two extruded steel pole 
pieces which are press-fitted on to the rotor shaft to 
sandwich a field coil and thus form the core of the 
electromagnet. Each pole piece has six "fingers" 
which, in position, mesh but do not touch. The field 
coil is connected to the slip rings which are also 
press-fitted on to the rotor shaft and supplied, via the 
brushes, with direct current from the aircraft's system. 

The stator is made up of a number of steel stamp- 
ings riveted together to form the core around which 
the three star-connected phase coils are wound. One 
end of each winding is connected to the bridge 
rectifier assembly while the other ends are joined 
together to form the neutral point. The stator 
assembly is clamped between the end frames. 

Cooling of the generator is provided by a fan at the 
driving end and by air passing through slotted vents in 
the slip-ring end frame. Heat at the silicon diodes is 
dissipated by mounting them on steel plates known 
as "heat sinks". 

Constant Frequency Systems 

In the development of electrical power supply systems, 
notably for large aircraft, the idea was conceived of 
an "all a.c." system, i.e. a primary generating system 
to meet all a.c. supply requirements, in particular 
those of numerous consumer services dependent on 
constant-frequency, and to meet d.c. supply require- 
ments via transformer and rectifier systems. 

One of the problems which had been a major 
stumbling block to the most effective use of a.c. 
generators was the problem of controlling frequency 
to permit several generators to operate in parallel. 
Parallel operation is, of course, of the highest impor- 
tance in system reliability, and the frequency must be 
constant within fairly narrow limits if full advantage of 
the system is to be obtained. 

At this stage of practical applications, a constant 
frequency is inherent in an a.c. system only if the 
generator is driven at a constant speed. The engines 
cannot be relied upon to do this directly and, as we 
have already learned, if a generator is connected directly 
to the accessory drive of an engine the output frequenc> 
will vary with engine speed. Some form of conversion 
equipment is therefore required and the type most 
widely adopted utilizes a transmission device inter- 
posed between the engine and generator, and which 
incorporates a variable-ratio drive mechanism. As an 
example we may consider the constant speed drive 
unit (C.S.D.) shown in Fig. 3.10, which is based on the 


Engine drive 

Generator mounting 
and coupling 

Fig 3.10 

Constant speed unit 

Sundstrand design and is in use in several current types 
of turbojet-powered aircraft. It employs a hydro- 
mechanical variable-ratio drive which, in its basic form, 
consists of a variable-displacement swash plate type of 
hydraulic pump and a constant displacement swash 
plate type of motor. The oil for system operation is 
supplied by charge pumps and governor systems fed 
from a reservoir which is pressurized by air tapped 
from the low-pressure compressor of the engine. Power 
from the engine is transmitted through an input shaft 
and gears, to a hydraulic cylinder block common to 
both pump and motor, and by the action of the 
internal hydraulic system, is finally transmitted to 
the motor and output gears and shaft coupled to the 
generator. The principle is illustrated very simply in 
Fig. 3.11. 

Mow shaft 

Drive t'Om engine 
/ Cylinder t>ock 

Swash picte 

Generator drive / f 
output shaft / 

/ Swash 

Centre ptate 

,vnt valve '.' H 

Control valve 

Overanve (JnaerO'^e 
pressure pressure 

Fig 3. II 

Principle of constant speed drive unit 

When the engine output is exactly equal to the 
required generator speed, the oil pressure and flow 
within the hydraulic system are such that the motor 

is hydraulically locked to the cylinder block and they 
rotate together; thus, the whole transmission system 
acts as a fixed coupling. If, however, there is a change 
in engine and input shaft speed, the governor system 
senses this and applies a greater or smaller pressure to 
the pump to vary the angle of its swash-plate. 

For example, if engine output is slower than the 
required generator speed, called an "overdrive" con- 
dition, the pressure increases; conversely, in an 
"underdrive" condition when engine output is faster, 
the pressure decreases. 

Variations in the angle of the swash-plate also varies 
the stroke of the pump pistons as they go round with 
the cylinder block, so that either a greater or smaller 
(underdrive) pressure is transmitted to the motor 
pistons. The motor pistons in turn exert a greater or 
smaller pressure on the motor swash-plate assembly 
made up of two stationary plates which sandwich an 
eccentric centre plate coupled to the output shaft, and 
free to rotate against ball bearings. Thus, assuming that 
an overdrive condition arises an increased pressure will 
be exerted by the motor pistons on the centre plate 
and there will be a tendency for it to be squeezed out 
from between the plates. However, since the plate is 
restrained to rotate independently about a fixed axis 
it will do so relative to the cylinder block, and at a 
faster rate, thereby overcoming the tendency for the 
engine to slow the generator down. In an underdrive 
condition, the pressure on the eccentric centre plate 
is decreased so that it will rotate at a slower rate 
relative to the cylinder block. 

The use of brushes and slip-rings for conveying 
excitation current to a.c. generator field windings 
presents similar problems to those associated with 
d.c. machines and although not quite as severe, because 
of the lower values of d.c. carried, the elimination of 
rotating surfaces in contact is desirable. Thus, the 
brushless a.c. generator was conceived, and although 
not specifically limited to constant-frequency systems, 
it went into commercial service in the mid-fifties 
with such a system, and is now normally associated 
with those systems employed in several current aircraft 

A sectioned view of typical generator is illustrated 
in Fig. 3.12. It consists of three principal components: 
a.c. exciter which generates the power for the main 
generator field; rotating rectifier assembly mounted 
on, and rotating with, the rotor shaft to convert the 
exciter output to d.c; and the main generator. All 


three components are contained within a cast alumin- 
ium casing made up of an end bell section and a stator 
frame section; both sections are secured externally by 
screws. A mounting flange, which is an integral part 
of the stator frame, carries twelve slots reinforced by 
steel inserts, and key-hole shaped to facilitate attach- 
ment of the generator to the mounting studs of the 
constant-speed drive unit. 

The exciter, which is located in the end bell section 
of the generator casing, comprises a stator and a three- 
phase star-wound rotor or exciter armature. The exciter 
armature is mounted on the same shaft as the main 
generator rotor and the output from its three-phase 
windings is fed to the rotating rectifier assembly. 

The rotating rectifier assembly supplies excitation 
current to the main generator rotor field coils, and 
since together with the a.c. exciter they replace the 
conventional brushes and slip rings, they thereby elimin- 
ate the problems associated with them. The assembly 
is contained within a tubular insulator located in the 
hollow shaft on which the exciter and main generator 
rotors are mounted; located in this manner they are 

close to the axis of rotation and are not, therefore, 
subjected to excessive centrifugal forces. A suppression 
capacitor is also connected in the rectifier circuit and 
is mounted at one end of the rotor shaft. Its purpose 
is to suppress voltage "spikes" created within the 
diodes under certain operating conditions. 

The main generator consists of a three-phase star- 
wound stator, and an eight-pole rotor and its associ- 
ated field windings which are connected to the output 
of the rotating rectifier. The leads from the three 
stator phases are brought directly to the upper surface 
of an output terminal board, thus permitting the air- 
craft wiring to be clamped directly against the phase 
leads without current passing through the terminal 
studs. In addition to the field coils, damper (amort- 
isseur) windings are fitted to the rotor and are located 
in longitudinal slots in the pole faces. Large copper 
bands, under steel bands at each end of the rotor 
stack, provide the electrical squirrel-cage circuit. The 
purpose of the damper windings is to provide an 
induction motor effect on the generator whenever 
sudden changes in load or driving torque tend to cause 

„ . Exciter terminal Fxriter 
«... , Output terminal cxuicr 

Output teminal board and cover ___, terminal 
boord and cover 

Thermostatic switch 

Cooling air screen^ 

Damper bor 

Rotating field 




Exciter shunl field and 
stabilizing windings 

Exciter mom poles 

Exciter armature 
Slotted rotor 


Exciter stator 



End bell 
AC stator 

Rotating field pole 



Stotor frame 

Output windings 
Damper windings 
Steel band 
Copper band 

Fig 3.12 

Brushless type a.c. generator 


the rotor speed to vary above or below the normal or 
synchronous system frequency. In isolated generator 
operation, the windings serve to reduce excessively 
high transient voltages caused by line-to-line system 
faults, and to decrease voltage unbalance, during 
unbalanced load conditions. In parallel operation (see 
p. 42), the windings also reduce transient voltages 
and assist in pulling in, and holding, a generator in 

The drive end of the main rotor shaft consists of a 
splined outer adaptor which fits over a stub shaft 
secured to the main generator rotor. The stub shaft, in 
turn, fits over a drive spindle fixed by a centrally 
located screw to the hollow section of the shaft con- 
taining the rotating rectifier assembly. The complete 
shaft is supported at each end by pre-greased sealed 

The generator is cooled by ram air which enters 
through the end bell section of the casing and passes 
through the windings and also through the rotor shaft 
to provide cooling of the rectifier assembly. The air 
is exhausted through a perforated screen around the 
periphery of the casing and at a point adjacent to the 
main generator stator. A thermally-operated overheat 
detector switch is screwed directly through the stator 

frame section into the stator of the main generator, 
and is connected to an overheat warning light on the 
relevant system control panel. 


The production of a desired output by any type of 
generator requires a magnetic field to provide excita- 
tion of the windings for starting and for the sub- 
sequent operational running period. In other words, 
a completely self-starting, self-exciting sequence is 
required. In d.c. generators, this is achieved in a fairly 
straightforward manner by residual magnetism in the 
electromagnet system and by the build up of cur-ent 
through the field windings. The field current, as it is 
called, is controlled by a voltage regulator system. 
The excitation of a.c. generators, on the other hand, 
involves the use of somewhat more complex circuits 
the arrangements of which are essentially varied to 
suit the particular type of generator and its control- 
ling system. However, they all have one common 
feature, i.e. the supply of direct current to the field 
windings to maintain the desired a.c. output. 


Figure 3. 1 3 is a schematic illustration of the method 

D.C. from bus-bar 
Rectified o.c. 


208-V a.c. 





±-H J H# 






Fig 3.13 
Generator excitation 


adopted for the generator illustrated in Fig. 3.7. In 
this case, excitation of the rotor field is provided by 
d.c. from the aircraft's main busbar and by rectified 
a.c. The principal components and sections of the 
control system associated with excitation are: the 
control switch, voltage regulation section, field 
excitation rectifier and current compounding section 
consisting of a three-phase current transformer and 

The primary windings of the compounding trans- 
former are in series with the three phases of the 
generator and the secondary windings in series with 
the compounding rectifier. 

When the control switch is in the "start" position, 
d.c. from the main busbar is supplied to the slip-rings 
and windings of the generator rotor; thus, with the 
generator running, a rotating magnetic field is set up 
to induce an alternating output in the stator. The 
output is tapped to feed a magnetic amplifier type of 
voltage regulator which supplies a sensing current 
signal to the excitation rectifier (see p. 38). When 
this signal reaches a pre-determined off-load value, the 
rectified a.c. through the rotor winding is sufficient 
for the generator to become self-excited and indepen- 
dent of the main busbar supply which is then dis- 

The maximum excitation current for wide-speed- 
range high-output generators of the type shown in 
Fig. 3.7 is quite high, and the variation in excitation 
current necessary to control the output under varying 
"load" conditions is such that the action of the voltage 
regulator must be supplemented by some other medium 
of variable excitation current. This is provided by the 
compounding transformer and rectifier, and by con- 
necting them in the manner already described, direct 
current proportional to load current is supplied to the 
rotor field windings. 

Fig. 3.14 illustrates the circuit diagram of the 
generator shown in Fig. 3.9 (see p. 32). When d.c. 
is switched on to the generator, the rotor field coil is 
energized and the pole piece "fingers" become 
alternately north and south magnetic poles. As the 
rotor rotates, the field induces a three-phase a.c. 
within the stator which is fed to the diodes and thence 
to the aircraft's system as rectified a.c. The level of 
voltage is regulated by a transistorized type of 
voltage regulator (see p. 39). 

The exciter stator of the generator described on page 
33 is made up of two shunt field windings, a 
stabilizing winding and also six permanent magnets; 

To services 


Bollery or 
externol power 

Fig 3.14 

Circuit diagram frequency- wild generator 


the latter provide a residual magnetic field for initial 
excitation. A temperature-sensitive resistance element 
(thermistor) is located between two of the stator 
terminals to compensate for changes in shunt field 
resistance due to temperature variations. 

The stabilizing winding is wound directly over the 
shunt field windings, and with the permanent magnet 
poles as a common magnetic core, a transformer type 
of coupling between the two windings is thereby 
provided. The rectifier assembly consists of six silicon 
diodes separated by insulating spacers and connected 
as a three-phase full-wave bridge. 

The excitation circuit arrangement for the generator 
is shown schematically in Fig. 3.15. When the generator 
starts running, the flux from the permanent magnets 
of the a.c. exciter provides the initial flow of current 
in its rotor windings. As a result of the initial current 
flow, armature reaction is set up, and owing to the 
position of the permanent magnetic poles, the reaction 
polarizes the main poles of the exciter stator in the 
proper direction to assist the voltage regulator in tak- 
ing over excitation control. 

The three-phase voltage produced in the windings is 
supplied to the rectifier assembly, the d.c. output of 
which is, in turn, fed to the field coils of the main 
generator rotor as the required excitation current. A 
rotating magnetic field is thus produced which induces 

a three-phase voltage output in the main stator wind- 
ings. The output is tapped and is fed back to the shunt 
field windings of the exciter, through the voltage 
regulator system, in order to produce a field supple- 
mentary to that of the permanent magnets. In this 
manner the exciter output is increased and the main 
generator is enabled to build up its output at a faster 
rate. When the main output reaches the rated value, 
the supplementary electromagnetic field controls the 
excitation and the effect of the permanent magnets is 
almost eliminated by the opposing armature reaction. 
During the initial stages of generator operation, 
the current flow to the exciter only passes through 
one of the two shunt field windings, due to the inverse 
temperature/resistance characteristics of the thermistor. 
As the temperature of the winding increases, the 
thermistor resistance decreases to allow approximately 
equal current to flow in both windings, thus maintain- 
ing a constant effect of the shunt windings. 

In the event that excitation current should suddenly 
increase or decrease as a result of voltage fluctuations 
due, for example, to switching of loads, a current will 
be induced in the stabilizing winding since it acts as a 
transformer secondary winding. This current is fed into 
the voltage regulator as a feedback signal to so adjust 
the excitation current that voltage fluctuations resulting 
from any cause are opposed and held to a minimum. 

Q V ' 

Permanent magnets 

— [ Current transformer 
— t Output 

sensing and 


To main o.c. bus bars 

Power transformer 
ond load 
magnetic amplifier 

Voltage regulator 

Exciter output 
Rectified a. c. excitation 
Main o-C. output 
Regulated excitation current 
ExcilotiOn current under 
fault condition 

Stabilizing feedback signal 

Fig 3.15 

Circuit diagram of brushless generator 



The control of the output voltages of a.c. generators is 
also an essential requirement, and from the foregoing 
description of excitation methods, it will be recognized 
that the voltage regulation principles adopted for d.c. 
generators can also be applied, i.e. automatic adjust- 
ment of excitation current to meet changing conditions 
of load and/or speed. Voltage regulators normally form 
part of generator system control and protection units. 

Frequency- Wild Generators Figure 3. 1 6 is a block 
functional diagram of the method used for the 
voltage regulation of the generator illustrated in Fig. 
3.7. Regulation is accomplished by a network of 
magnetic amplifiers or transducers, transformers and 
bridge rectifiers interconnected as shown. In addition 
to the control of load current delivered by the gener- 
ator, a further factor which will affect control of 
field excitation is the error between the line voltage 
desired and the actual voltage obtained. As already 
explained on page 36, the compounding transformer 
and rectifier provides excitation current proportional 
to load current, therefore the sensing of error voltages 
and necessary re-adjustment of excitation current 
must be provided by the voltage regulation network. 
It will be noted from the diagram that the three- 
phase output of the generator is tapped at two points; 
at one by a three-phase transformer and at the other 
by a three-phase magnetic amplifier. The secondary 
winding of one phase of the transformer is connected 

to the a.c. windings of a single-phase "error sensing" 
magnetic amplifier and the three primary windings 
are connected to a bridge "signal" rectifier. The d.c. 
output from the rectifier is then fed through a voltage- 
sensing circuit made up of two resistance arms, one 
(arm "A") containing a device known as a barretter 
the characteristics of which maintain a substantially 
constant current through the arm, the other (arm 
"B") of such resistance that the current flowing through 
it varies linearly with the line voltage. The two current 
signals, which are normally equal at the desired line 
voltage, are fed in opposite directions over the a.c. 
output windings in the error magnetic amplifier. When 
there is a change in the voltage level, the resulting 
variation in current flowing through arm "B" un- 
balances the sensing circuit and, as this circuit has the 
same function as a d.c. control winding, it changes the 
reactance of the error magnetic amplifier a.c. output 
windings and an amplified error signal current is pro- 
duced. After rectification, the signal is then fed as d.c. 
control current to the three-phase magnetic amplifier, 
thus causing its reactance and a.c. output to change 
also. This results in an increase or decrease, as appro- 
priate, of the excitation current flow to the generator 
rotor field winding, continuing until the line voltage 
produces balanced signal conditions once more in the 
error sensing circuit. 

Fig. 3.17 shows the circuit arrangement of a typical 
transistorized voltage regulator as employed with the 
generator shown in Fig. 3.9. Before going into its 

Error sensing sionol 

Fig 3.16 
Voltage regulation - magnetic amplifier 



► Battery current 

► Rectified current 

— r» Reverse current 

Fig 3.17 

Transistorized voltage regulator 

operation, however, it will be useful to make a 

brief review of the primary function and fundamental 

characteristics # of the device known as the transistor. 

The primary function of a transistor is to "transfer 
resistance" within itself and depending on its con- 
nection within a circuit it can turn current "on" and 
"off and can increase output signal conditions; in 
other words, it can act as an automatic switching 
device or as an amplifier. It has no moving parts and is 
made up of three regions of a certain material, usually 
germanium, known as a semiconductor (see also p. 47) 
and arranged to be in contact with each other in some 
definite conducting sequence. Some typical transistor 
contact arrangements are shown in Fig. 3.18 together 
with the symbols used. The letters "p" and "n" 
refer to the conductivity characteristic of the 
germanium and signify positive-type and negative-type 
respectively. A transistor has three external 
connections corresponding to the three regions or 
elements, namely the emitter, the collector and the 

base, and as will be noted they contact each other in 
the sequence of either n-p-n or p-n-p. 

Let us consider the action of a transistor con- 
nected in the p-n-p arrangement. The emitter is 
positive with respect to base and the point contact 
between them causes a field of sufficiently high inten- 
sity for electrons to break out from the base and flow 
through the emitter, thus giving rise to a current flow 
in the direction shown. At the same time, and owing 
to the atomic structure of the semiconductor material, 
more positive carriers, or "holes" as they are termed, 
are formed and move towards the collector to produce 
a collector current. For a transistor connected in the 
n-p-n arrangement, a similar exchange of electrons and 
positive carriers would take place but in a manner 
which causes a current flow at the emitter and collector 
junction in the reverse direction. This briefly is trans- 
istor action and in correctly designed contact arrange- 
ments a change of current in one circuit, e.g. emitter- 
base, gives rise to a corresponding change of current in 





N P. 







Emitter Collector 

Electron flow • 'Holes' — — Current (low 

4f— *H 

♦ ♦ 


Emitter bias Collector bios Emitter bios Collector bios 

Emitter'V \/CoNeclor EnirtterN/ \/ Collector 


Fig 3.18 

Transistor contact arrangements 

the collector circuit and, furthermore, may be ampli- 

In the regulator circuit shown in Fig. 3.17, the 
three transistors (TR,, TR 2 and TR 3 ) are connected 
in the n-p-n arrangement. When the system control 
switch is "on", excitation current flows initially from 
the battery to the base of TR 2 and through a voltage 
dividing network made up of resistances R,, R 2 and 
RV, . The purpose of this network in conjunction with 
the Zener diode "Z" (see also p. 49) is to establish 
the system-operating voltage. With power applied to 
the base of TR 2 , the transistor is switched on and 
battery current flows to the collector and emitter 
junction. The amplified output in the emitter circuit 
flows to the base of TR 3 thereby switching it on so 
that the battery current supplied to the field winding 
can be conducted to ground via the collector-emitter 
junction of TR 3 . When the generator is running, the 
rotating magnetic field induces an alternating current 
in the stator and this is rectified and supplied to the 
d.c. power system of the aircraft. 

When the generator output voltage reaches the pre- 
set operating value, the current flowing in the reverse 
direction through the Zener diode causes it to break- 
down and to allow the current to flow to the base of 
TRj thus switching it on. The collector-emitter junc- 
tion of TR! now conducts, thereby diverting current 

away from the base of TR 2 and switching it off. This 
action, in turn, switches off TR 3 and so excitation 
current to the generator field winding is cut-off. The 
rectifier across the field winding (D,) provides a path 
so that field current can fall at a slower rate and thus 
prevent generation of a high voltage at TR 3 each time 
it is switched off. 

When the generator output voltage falls to a value 
which permits the Zener diode to cease conduction, 
TR, will again conduct to restore excitation current 
to the field winding. This sequence of operation is 
repeated and the generator output voltage is thereby 
maintained at the preset operating value. 

The regulation of the output of a constant-frequency 
system is also based on the principle of controlling 
field excitation, and some of the techniques thus far 
described are in many instances applied. In installa- 
tions requiring a multi-arrangement of constant-fre- 
quency generators, additional circuitry is required to 
control output under load-sharing or parallel operating 
conditions and as this control also involves field excita- 
tion, the overall regulation circuit arrangement is of 
an integrated, and sometimes complex, form. At this 
stage, however, we are only concerned with the funda- 
mental method of regulation and for this purpose 
we may consider the relevant sections or stages of a 
typical circuit shown schematically in Fig. 3.19. 

The circuit is comprised of three main sections: a 
voltage error detector, pre-amplifier and a power 
amplifier. The function of the voltage error detector 
is to monitor the generator output voltage, compare 
it with a fixed reference voltage and to transmit any 
error to the pre-amplifier. It is made up of a three- 
phase bridge rectifier connected to the generator out- 
put, and a bridge circuit of which two arms contain 
gas-filled regulator tubes and two contain resistances. 
The inherent characteristics of the tubes are such that 
they maintain an essentially constant voltage drop 
across their connections for a wide range of current 
through them and for this reason they establish the 
reference voltage against which output voltage is con- 
tinuously compared. The output side of the bridge is 
connected to an "error" control winding of the pre- 
amplifier and then from this amplifier to a "signal" 
control winding of a second stage or power amplifier. 
Both stages arc three-phase magnetic amplifiers. The 
final amplified signal is then supplied to the shunt 
windings of the generator a.c. exciter stator (see also 
Fig. 3.15). 


Gene rotor 

Error delector 

Fig 3.19 
Constant frequency system voltage regulation 

The output of the bridge rectifier in the error 
detector is a d.c. voltage slightly lower than the aver- 
age of the three a.c. line voltages; it may be adjusted 
by means of a variable resistor (RV,) to bring the 
regulator system to a balanced condition for any 
nominal value of line voltage. A balanced condition of 
the bridge circuit concerned is obtained when the 
voltage applied across the bridge (points "A" and "B") 
is exactly twice that of the voltage drop across the 
two tubes. Since under this condition, the voltage drop 
across resistors R t and R 2 will equal the drop across 
each tube, then no current will flow in the output cir- 
cuit to the error control winding of the pre-amplifier. 

If the a.c. line voltage should go above or below the 
fixed value, the voltage drops across R t and R 2 will 
differ causing an unbalance of the bridge circuit and a 
flow of current to the "error" control winding of the 
pre-amplifier. The direction and magnitude of current 
flow will depend on whether the variation, or error in 
line voltage, is above (positive error signal) or below 
(negative error signal) the balanced nominal value, and 
on the magnitude of the variations. 

When current flows through the "error" control 
winding the magnetic flux set up alters the total flux 
in the cores of the amplifier, thereby establishing a 
proportional change in the amplifier output which is 

applied to the signal winding of the power amplifier. 
1 f the error signal is negative it will cause an increase 
in core flux, thereby increasing the power amplifier 
output current to the generator exciter field winding. 
For a positive error signal the core flux and excitation 
current will be reduced. Thus, the generator output is 
controlled to the preset value which on being attained 
restores the error detector bridge circuit to the 
balanced condition. 

Load Sharing or Paralleling 

Frequency-Wild Systems. In systems of this type, the 
a.c. output is supplied to independent consumer equip- 
ment and since the frequency is allowed to go uncon- 
trolled, then paralleling or sharing of the a.c. load is 
not possible. In most applications this is by design; for 
example, in electrical de-icing equipment utilizing 
resistance type heaters, a variable frequency has no 
effect on system operation; therefore reliance is placed 
more on generator dependability and on the simplicity 
of the generating system. In rectified a.c. systems fre- 
quency is also uncontrolled, but as most of the output 
is utilized for supplying d.c. consumer equipment, 
load sharing is more easily accomplished by paralleling 
the rectified output through equalizing circuits in a 


similar manner to that adopted for d.c. generating 
systems (see p. 13). 

Constant-Frequency Systems. These systems are 
designed for operation under load sharing or paralleling 
conditions and in this connection regulation of the two 
parameters, real load and reactive load, is required. Real 
load is the actual working load output in kilowatts (kW) 
available for supplying the various electrical services, 
and the reactive load is the so-called "wattless load" 
which is in fact the vector sum of the inductive and 
capacitive currents and voltage in the system expressed 
in kilovolt-amperes reactive (WAR). (See Fig. 3.6 
once again.) 

Since the real load is directly related to the input 
power from the prime mover, i.e. the aircraft engine, 
real-load sharing control must be on the engine. There 
are, however, certain practical difficulties involved, but 
as it is possible to reference back any real load un- 
balance to the constant-speed drive unit between 
engine and generator, real-load sharing control is 
effected at this unit by adjusting torque at the output 
drive shaft. 

Reactive load unbalances are corrected by control- 
ling the exciter field current delivered by the voltage 
regulators to their respective generators, in accordance 
with signals from a reactive load sharing circuit. 


The sharing of real load between paralleled generators 
is determined by the real relative rotational speeds of 
the generators which in turn influence the voltage 
phase relationships. 

As we learned earlier (see p. 32) the speed of a 
generator is determined by the initial setting of the 
governor on its associated constant speed drive. It is 
not possible, however, to attain exactly identical 
governor settings on all constant speed drives employed 
in any one installation, and so automatic control of 
the governors becomes necessary. 

A.C. generators are synchronous machines. There- / 
fore when two or more operate in parallel they lock 
together with respect to frequency and the system 
frequency established is that of the generator whose 
output is at the highest level. Since this is controlled 
by speed-governing settings then it means that the 
generator associated with a higher setting will carry 
more than its share of the load and will supply energy 
which tends to motor the other maclunes in parallel 
with it. Thus, sharing of the total real load is unbal- 
anced, and equal amounts of energy in the form of 

torque on the generator rotors must be supplied. 

Fundamentally, a control system is comprised of 
two principal sections: one in which the unbalance is 
determined by means of current transformers, and 
the other (load controlling section) in which torques 
are established and applied. A circuit diagram of the 
system as applied to a four-generator installation is 
shown schematically in Fig. 3.20. 

The current transformers sense the real load dis- 
tribution at phase "C" of the supply from each gener- 
ator, and are connected in series and together they 
form a load sharing loop. Each load controller is 
made up of a two-stage magnetic amplifier controlled 
by an error sensing element in parallel with each cur- 
rent transformer. The output side of each load con- 
troller is, in turn, connected to a solenoid in the speed 
governor of each constant speed unit. 

When current flows through phase "C" of each 
generator a voltage proportional to the current is 
induced in each of the current transformers and as 
they are connected in series, then current will flow in 
the load sharing loop. This current is equal to the 
average of the current produced by all four trans- 

Let us assume that at one period of system opera- 
tion, balanced load sharing conditions are obtained 
under which the current output from each transformer 
is equal to five amps, then the average flowing in the 
load sharing loop will be five amps, and no current cir- 
culates through the error sensing elements. If now a 
generator, say No. 1 , runs at a higher speed governor 
setting than the other three generators, it will carry 
more load and will increase the output of its associ- 
ated current transformer. 

The share of the load being carried by the other 
generators falls proportionately, thereby reducing the 
output of their current transformers and the average 
current flowing in the load sharing loop remains the 
same, i.e. five amps. If, for example, it is assumed that 
the output of No. 1 generator current transformer is 
increased to eight amps a difference of three amps 
will flow through the error sensing element of its 
relevant load controller. The three amps difference 
divides equally between the other generators and so 
the output of each corresponding current transformer 
is reduced by one amp, a difference which flows 
through the error sensing elements of the load con- 
trollers. The error signals are then applied as d.c. 
control signals to the two-stage magnetic amplifiers 
and are fed to electromagnetic coils which are mounted 
adjacent to permanent magnet flyweights and form 




ornplif jer 


"c s^ees 

T c &PMO 

To sseea 

Fig 3.20 

Real-load sharing 

part of the governor in each constant speed drive unit. 
The current and magnetic field simulate the effects of 
centrifugal forces on the flyweights and are of such 
direction and magnitude as to cause the flyweights 
to be attracted or repelled. 

Thus, in the unbalanced condition we have assumed, 
i.e. No. 1 generator running at a higher governor set- 
ting, the current and field resulting from the error 
signal applied to the corresponding load controller 
flows in the opposite sense and repels the flyweights, 
thereby simulating a decrease of centrifugal force. 
The movement of the flyweights causes oil to flow to 
underdrive and the output speed of the constant speed 
unit drive decreases, thereby correcting the governor 
setting to decrease the load being taken by No. 1 
generator. The direction of the current and field in 
the load controller sensing elements of the remaining 
generators is such that the governor flyweights in 
their constant speed drive units are attracted, allowing 
oil to flow to overdrive, thereby increasing the load 
being taken by each generator. 

The sharing of reactive load between paralleled gen- 
erators depends on the relative magnitudes of their out- 
put voltages which vary, and as with all generator 
systems are dependent on the settings of relevant 
voltage regulators and field excitation current (see 
also p. 35). If, for example, the voltage regulator of 
one generator is set slightly above the mean value of 
the whole parallel system, the regulator will sense an 
under-voltage condition and it will accordingly increase 
its excitation current in an attempt to raise the whole 
system voltage to its setting. However, this results in 
a reactive component of current flowing from the 
"over-excited" generator which flows in opposition to 
the reactive loads of the other generators. Thus, its 
load is increased while the loads of the other generators 
are reduced and unbalance in reactive load sharing 
exists. It is therefore necessary to provide a circuit to 
correct this condition. 

In principle the method of operation of the reactive 
load-sharing circuit is similar to that adopted in the . 


real load sharing circuit described earlier. A difference 
in the nature of the circuitry should however be noted 
at this point. Whereas in the real-load sharing circuit 
the current transformers are connected directly to 
the error detecting elements in load controlling units, 
in a reactive-load sharing circuit (see Fig. 3.21) they 
are connected to the primary windings of devices 
called mutual reactors. These are, in fact, transformers 
which have (i) a power source connected to their 
secondary windings in addition to their primaries; in 
this instance, phase "C" of the generator output, and 
(ii) an air gap in the iron core to produce a phase 
displacement of approximately 90 degrees between 
the primary current and secondary voltage. They 
serve the purpose of delivering signals to the voltage 
regulator which is proportional to the generator's 
reactive load only. 

When a reactive-load unbalance occurs, the current 
transformers detect this in a similar manner to those 
associated with the real-load sharing circuit and they 
cause differential currents to flow in the primary 
windings of their associated mutual reactors. Voltages 
proportional to the magnitude of the differential 

currents are induced in the secondary windings and 
will either lead or lag generator current by 90 degrees. 
When the voltage induced in a particular reactor 
secondary winding leads the associated generator 
current it indicates that a reactive load exists on the 
generator; in other words, that it is taking more than 
its share of the total load, fn this condition, the 
voltage will add to the voltage sensed by the secon- 
dary winding at phase "C". If, on the other hand, the 
voltage lags the generator current then the generator 
is absorbing a reactive load, i.e. it is taking less share 
of the total load and the voltage will subtract from 
that sensed at phase "C". 

The secondary winding of each mutual reactor is 
connected in series with an error detector in each 
voltage regulator, the detector functioning in the 
same manner as those used for voltage regulation 
and real-load sharing (see pp. 40 and 42). 

Let us assume that No. 1 generator takes the 
greater share of the load, i.e. it has become over- 
excited. The voltage induced in the secondary wind- 
ing of the corresponding mutual reactor will be 
additive and so the error detector will sense this as 

. A 



t Gen. 
1 3 



( Gen V 

l C pmr> 

\ 8 

( Gen. \B 

V 6 f"nf"» 

/c ,„ 







T~ " 

* T 



I _ 

1 r 



' — 






1 — ^smssb — 

Mutuol reactors 

fOoWT — 


To pre-omplifier 
and generator 
shunt field 

f 00000 ' — 

fCoTOT — 




(• 00000^ 


To pre-omplifier 
and generotor 
shunt field 

To pre-amphfier 
and generofor 
shunt field 

To pre -amplifier 
and generator 
shunt field 

Fig 3.21 

Reactive-load sharing 


an overvoltage. The resulting d.c. error signal is 
applied to the pre-amplifier and then to the power 
amplifier the output of which is adjusted to reduce 
the amount of exciter current being delivered to the 
No. 1 generator. In the case of the other three 
generators they will have been carrying less than 
their share of the reactive load and, therefore, the 
voltages induced in their mutual reactors will have 
lagged behind the currents from the generators, 
resulting in opposition to the voltages sensed by 
the secondary windings. Thus, the output of each 
power amplifier will be adjusted to increase the 
amount of exciter current being delivered to their 
associated generators until equal reactive load-sharing 
is restored between generators within the prescribed 

The application of generators dependent upon an 
airstream as the prime mover is by no means a 
new one and, having been adopted in many early 
types of aircraft for the generation of electrical 
power, the idea of repeating the practice for to-day's 
advanced electrical systems would, therefore, seem 
to be a retrogressive step. However, an air-drive can 
serve as a very useful stand-by in the event of failure 
of a complete main a.c. generating system and it is 
in this emergency role that it is applied to several 
types of aircraft. 

The drive consists of a two-bladed fan or air- 
turbine as it is sometimes called, and a step-up ratio 
gear train which connects the fan to a single a.c. 
generator. The generator is of a similar type to the 
main generator (see also p. 33) but has a lower 
output rating since it is only required to supply the 
consumer equipment essential under emergency con- 
ditions. The complete unit is stowed on a special 
mounting in the aircraft fuselage, and when required 
is deployed by a mechanically linked release handle 
in the flight compartment. When deployed at air- 
speeds of between 120 to 430 knots, the fan and 
generator are driven up to their appropriate speeds by 
the airstream, and electrical power is delivered via a 
regulator at the rated values. A typical nominal fan 
speed is 4,800 rev/min and is self-governed by varying 
the blade pitch angles. The gearbox develops a gener- 
ator shaft speed of 12,000 rev/min. After deployment 
of the complete unit, it can only be restowed on the 

Test Questions 

1. The frequency of an alternator may be deter- 
mined by: 

(a) dividing the number of phases by the voltage. 

(b) multiplying the number of poles by 60 and 
dividing by the rev/min. 

(c) multiplying the rev/min by the number of 
pairs of poles and dividing by 60. 

2. Explain the term r.m.s. value. 

3. The current in a purely capacitive circuit will: 

(a) lead the applied voltage. 

(b) lag the applied voltage. 

(c) be in phase with the applied voltage. 

4. (a) With the aid of circuit diagrams briefly 

describe the two methods of interconnecting 
(b) State the mathematical expressions for cal- 
culating line voltage and line current in each 

5. Explain the term Power Factor and state how it 
is affected by a circuit containing inductance and 

6. What do you understand by the term "frequency- 
wild system"? 

7. State the factors upon which the frequency output 
of an a.c. generator depend. With the aid of a 
sketch, describe the construction of a typical 
aircraft generator and determine mathematically 
the output frequency of the machine which you 

have illustrated. 


8. With the aid of a schematic diagram, describe how 
a generator can be excited and how its output 
voltage can be controlled. 

9. Describe how the speed of a constant-frequency 
generator is maintained. 

10. With the aid of a sketch, explain the construction 
and operating principles of a three-phase brush- 
less generator. 


1 1 . What factors must be controlled when constant- 
frequency a.c. generators are operated in 

12. What is the meaning of kVAR and to which of 
the factors does it refer? 

13. State the functions which current transformers 
perform in controlling load sharing between 
constant-frequency generators. 

1 4. By what means does a constant speed drive 
sense an underspeed or overspeed? 


1 5. What is a mutual reactor and in which section of 17. Name the three regions or elements of a trans- 
a load-sharing circuit is it used? istor and state the function of each. 

16. In connection with a transistor, what do the 18. By means of simple diagrams show the two 
letters "p" and "n" refer to? principal contact sequences of transistors. 


Power Conversion Equipment 

In aircraft electrical installations a number of different 
types of consumer equipment are used which require 
power supplies different from those standard supplies 
provided by the main generator. For example, in an 
aircraft having a 28 volts d.c. primary power supply, 
certain instruments and electronic equipment are 
employed which require 26 volts and 1 15 volts a.c. 
supplies for their operation, and as we have already 
seen, d.c. cannot be entirely eliminated in aircraft 
which are primarily a.c. in concept. Furthermore, we 
may also note that even within the items of consumer 
equipment themselves, certain sections of their cir- 
cuits require different types of power supply and/or 
different levels of the same kind of supply. It there- 
fore becomes necessary to employ not only equipment 
which will convert electrical power from one form to 
another, but also equipment which will convert one 
form of supply to a higher or lower value. 

The equipment required for the conversion of main 
power supplies can be broadly divided into two main 
types, static and rotating, and the fundamentals of 
construction and operation of typical devices and 
machines are described under these headings. 

Static Converting Equipment 

The principal items which may be grouped under this 
heading are rectifiers and transformers, some applic- 
ations of which have already been discussed in Chapter 
3, and static d.c./a.c. converters. 

The latter items are transistorized equivalents of 
rotary inverters and a description of their construction 
and operating fundamentals will be given at the end 
of this chapter. 


The process of converting an a.c. supply into a d.c. 

supply is known as rectification and any static appar- 

atus used for this purpose is known as a rectifier. 
The rectifying action is based on the principle 
that when a voltage is applied to certain combinations 
of metallic and non-metallic elements in contact with 
each other, an exchange of electrons and positive 
current carriers (known as "holes") takes place at 
the contact surfaces. As a result of this exchange, a 
barrier layer is formed which exhibits different 
resistance and conductivity characteristics and allows 
current to flow through the element combination 
more easily in one direction than in the opposite 
direction. Thus, when the applied voltage is an 
alternating quantity the barrier layer converts the 
current into a undircctional flow and provides a 
rectified output. 

One of the elements used in combination is 
referred to as a "semi-conductor" which by definition 
denotes that it possesses a resistivity which lies 
between that of a good conductor and a good insul- 
ator. Semi-conductors are also further defined by 
the number of carriers, i.e. electrons and positive 
"holes", provided by the "crystal lattice" form of the 
element's atomic structure. Thus, an element having 
a majority of electron carriers is termed "n-type" 
while a semi-conductor having a majority of "holes" is 
termed "p-type". 

If a p-type semi-conductor is in contact with a 
metal plate as shown in Fig. 4. 1 , electrons migrate from 
the metal to fill the positive holes in the semi-conductor, 
and this process continues until the transference of 
charge has established a p.d. sufficient to stop it. By 
this means a very thin layer of the semi-conductor is 
cleared of positive holes and thus becomes an effective 
insulator, or barrier layer. When a voltage is applied 
such that the semi-conductor is positive with respect 
to the metal, positive holes migrate from the body of 
the semi-conductor into the barrier layer, thereby 
reducing its "forward" resistance and restoring con- 


ductivity. If, on the other hand, the semi-conductor 
is made negative to the metal, further electrons are 
drawn from the metal to fill more positive holes and 
the "reverse" resistance of the barrier layer is thus 
increased. The greater the difference in the resistance 
to current flow in the two directions the better is the 
rectifying effect. 

Barrier loyer 


P-Type semiconductor 


© E'ectrons 
@ Positive 'holes' 
(—) E lectron occeptor atoms 

Fig 4.1 

Semi-conductor/Metal junction 

A similar rectifying effect is obtained when an n- 
type semi-conductor is in contact with metal and a 
difference of potential is established between them, 
but in this case the direction of "easy" current flow 
is reversed. In practice, a small current does flow 
through a rectifier in the reverse direction because 
p-type material contains a small proportion of free 
electrons and n-type a small number of positive holes. 

In the rectification of main a.c. power supplies, 
rectifiers are now invariably of the type employing 
the p-type non-metallic semi-conductors, selenium 
and silicon. Rectifiers employing germanium (a 
metallic element) are also available but as their 
operating temperature is limited and protection 
against short duration overloads is difficult, they 
are not adopted in main power systems. 


The selenium rectifier is formed on an aluminium 
sheet which serves both as a base for the rectifying 
junction and as a surface for the dissipation of heat. 
A cross-section of an element is shown diagrammatic- 
ally in Fig. 4.2 and from this it will be noted that 

Rectifying junction 

4. frfe^ft^ 



Hole for 
mounlmg bolt 

Aluminium base 

Fig 4.2 

Cross-section of a selenium rectifier element 

the rectifying junction covers one side of the base with 
the exception of a narrow strip at the edges and a 
small area around the Fixing hole which is sprayed 
with a layer of insulating varnish. A thin layer of a low- 
melting point alloy, referred to as the counter 
electrode, is sprayed over the selenium coating and 
insulating varnish. Contact with the two elements of 
the rectifying junction, or barrier layer, is made 
through the base on one side and the counter elec- 
trode on the other. 

Mechanical pressure on the rectifying junction 
tends to lower the resistance in the reverse direction 
and this is prevented in the region of the mounting 
studs by the layer of varnish. 

In practice a number of rectifying elements may 
be connected in series or parallel to form what is 
generally referred to as a rectifier stack. Three 
typical stacks are shown in Fig. 4.3; the one in the 

Fig 4.3 

Typical rectifier stacks 


upper part of the illustration being used in a type of 
transformer rectifier unit. When connected in series 
the elements increase the voltage handling ability of 
a rectifier and when connected in parallel the ampere 
capacity is increased. 


Silicon rectifiers, or silicon junction diodes as they 
are commonly known, do not depend on such a large 
barrier layer as selenium rectifiers, and as a result they 
differ radically in both appearance and size. This will 
be apparent from Fig. 4.4 which illustrates a junction 
diode of a type similar to that used in the brushless 
generator described in Chapter 3. 

Anode leod 


Copper bose 




-Silicon wafer 

Threaded mounting 
stud and ca1 node 

Fig 4.4 

Silicon junction diode 

The silicon is in the form of an extremely small 
slice cut from a single crystal and on one face it has a 
fused aluminium alloy contact to which is soldered an 
anode and lead. The other face is soldered to a base, 
usually copper, which forms the cathode and at the 
same time serves as a heat sink and dissipator. The 
barrier layer is formed at the aluminium-silicon 

To protect the junction from water vapour and 
other deleterious materials, which can seriously 
impair its performance, it is mounted in a hermetically- 
sealed case. 

The limiting factors in the operation of a rectifier 
are: (i) the maximum temperature permissible and 
(ii) the minimum voltage, i.e. the reverse voltage, 
required to break down the barrier layer. In selenium 
rectifiers the maximum temperature is of the order 

of 70° C. For germanium the temperature is about 
50°C, while for silicon up to 1 50°C may be reached 
without destroying the rectifier. It should be noted 
that these figures represent the actual temperature at 
the rectifying junction and therefore the rectifier, as 
a complete unit, must be at a much lower temperature 
Proper cooling under all conditions is therefore an 
essential requirement and is normally taken care 
of by blower motors or other forced air methods such 
as the one adopted in the brushless generator described 
in Chapter 3. 

Voltage ratings are determined by the ability of a 
rectifier to withstand reverse voltage without passing 
excessive reverse current, and the characteristics are 
such that reverse current does not increase propor- 
tionately to the applied voltage. This is because once 
all the current carriers have been brought into action 
there is nothing to carry any further current. However, 
at a sufficiently high voltage the resistance in the 
reverse direction breaks down completely and reverse 
current increases very sharply. The voltage at which 
breakdown occurs is called the Zener voltage, and as 
it depends on the impurity content of the material 
used, a constant value can be chosen by design and 
during manufacture of a rectifier. For power rectifica- 
tion, rectifiers must have a high Zener voltage value 
and each type must operate at a reverse voltage below 
its designed breakdown value. Some rectifiers, how- 
ever, are designed to breakdown at a selected value 
within a low voltage range (between 2 and 40 volts is 
typical) and to operate safely and continuously at 
that value. These rectifiers are called Zener Diodes 
and since the Zener voltage is a constant and can there- 
fore serve as a reference voltage, they are utilized 
mostly in certain low voltage circuits and systems for 
voltage level sensing and regulation (see also p. 40). 

An S.C.R., or thyristor as it is sometimes called, is a 
development of the silicon diode and it has some of 
the characteristics of a thyratron tube. It is a three- 
terminal device, two terminals corresponding to those 
of an ordinary silicon diode and the third, called the 
"gate" and corresponding to the thyratron grid. The 
construction and operating characteristics of the 
device are shown in Fig. 4.5. The silicon wafer which 
is of the "n-type" has three more layers formed with- 
in it in the sequence indicated. 

When reverse voltage is applied an S.C.R. behaves 
in the same manner as a normal silicon diode, but 
when forward voltage is applied current flow is prac- 




WM M//M 



breakover voltage 

Device can be fired at any 
desired value of forward 
voltage by applying gate 


Gate connector 

Fig 4.5 

Silicon controlled rectifier 

tically zero until a forward critical "breakover" volt- 
age is reached. The voltage at which breakover takes 
place can be varied by applying small current signals 
between the gate and the cathode, a method known 
as "firing". Once conduction has been initiated it can 
be stopped only by reducing the voltage to a very low 
value. The mean value of rectified voltage can be 

controlled by adjusting the phasing of the gate signal 
with respect to the applied voltage. Thus, an S.C.R. 
not only performs the function of power rectification, 
but also the function of an on-off switch, and a vari- 
able power output device. A typical application of 
S.C.R. switching is in the battery charger unit already 
referred to on p. 25. Fig. 4.6 shows how an S.C.R. 


produces a variable dx. voltage which, for example, 
would be required in a variable speed motor circuit, 
as gate signal currents or "firing point" is varied. 

Average value 

Current flow 

AC input V, 

Average value 




Average value 



Fig 4.6 

Variable d.c output from a silicon controlled rectifier 


Rectifiers are used in single-phase and three-phase 
supply systems and, depending on the conversion 
requirements of a circuit or system, they may be 
arranged to give either half-wave or full-wave rectifica- 
tion. In the former arrangement the d.c. output is 
available only during alternate half-cycles of an a.c. 
input, while in the latter a d.c. output is available 
throughout a cycle. 

The single-phase half-wave circuit shown in Fig. 
4.7(a) is the simplest possible circuit for a rectifier 
and summarizes, in a practical manner, the operating 
principles already described. The output from the 
single rectifier is a series of positive pulses the number 
of which is equal to the frequency of the input voltage. 
For a single-phase a.c. input throughout a full cycle, a 
bridge connection of rectifiers is used (Fig. 4.7(b)). 
For half-wave rectification of a three-phase a.c. 
input the circuit is made up of three rectifiers in the 
manner shown in Fig. 4.8. This arrangement is com- 
parable to three single-phase rectification circuits, but 
since the positive half-cycles of the input are occur- 
ring at time intervals of one third of a cycle (120 
degrees) the number of d.c. pulses or the ripple fre- 
quency is increased to three times that of the supply 
and a smoother output waveform is obtained. 


J Output 



Current flow in l" half-cycle 
Current flow in 2" 1 holf -cycle 

Fig 4.7 
Single-phase rectification 

(a) Half-wave 

(b) Full-wave 

Figure 4.9 shows the circuit arrangement for the 
full-wave rectification of a three-phase a.c. input; it 
is of the bridge type and is most commonly used for 
power rectification in aircraft. Examples of three- 
phase bridge rectifier applications have already been 
shown in Chapter 3 but we may now study the circuit 
operations in a little more detail. 

—» Current flow prose I 
=^. .. ... 2 

=-» - ■ •• 3 

Fig 4.8 

Three-phase half-wave rectification 


O.C. output 

R 2 - R,. 

Phose I 

Phose 2 

Phase 3 

D.C. outpul + 

Line volts between Phoses I and 2 

Lne volts between Phoses I ond 3 

Between phoses 2 ond I , R 2 * ond R,- conduct 
Between phoses 3 ond t. R 5 * ond R,- conduct 
Between phoses 3 and 2, Rj« ana Rj- conduct 

Line volts between Phoses 2 ond 3 

Fig 4.9 

Operation of a full-wave bridge rectifier 

In this type of circuit only two rectifiers are con- 
ducting at any instant; one on the positive side and the 
other on the negative side. Also the voltage applied to 
the bridge network is that between two of the phases, 
i.e. the line voltage. Let us consider the points "A" 
and "B" on the three phase voltage curves. These 
points represent the line voltage between phases 1 
and 2 of the supply and from the circuit diagram we 
note that rectifiers Ri + and R 2 - only will conduct. 
From "B" to "C" the line voltage corresponds to that 
between phases 1 and 3 and R]+ now conducts in 
conjunction with R 3 ~. Between the points "C" and 
"D" the line voltage corresponds to that between 

phases 2 and 3 so that rectifier R 2 + now takes over 
and conducts in conjunction with R 3 — . This process 
continues through the remaining three conducting 
paths, the sequence of the relevant phases and the 
rectifiers which conduct being as tabulated in Fig. 4.9. 

The output voltage, which is determined by the 
distance between the positive and negative crests, con- 
sists of the peaks of the various line voltages for phase 
angles of 30 degrees on either side of their maxima. 
Since the negative half-cycles are included, then the 
ripple frequency of a bridge rectifier output is six 
times that of the a.c. input and an even smoother 
waveform is obtained. 



A transformer is a device for converting a.c. at one 
frequency and voltage to a.c. at the same frequency 
but at another voltage. It consists of three main parts: 
(i) an iron core which provides a circuit of low 
reluctance for an alternating magnetic field created 
by, (ii) a primary winding which is connected to the 
main power source and (iii) a secondary winding 
which receives electrical energy by mutual induction 
from the primary winding and delivers it to the 
secondary circuit. There are two classes of trans- 
formers, voltage or power transformers and current 

Principle. The three main parts are shown schem- 
atically in Fig. 4.10. When an alternating voltage is 
applied to the primary winding an alternating 
current will flow and by self-induction will establish 





winding a 

> winding 



Step- up ratio 

Step-down ratio 


Transformer principle 

a voltage in the primary winding which is opposite 
and almost equal to the applied voltage. The differ- 
ence between these two voltages will allow just enough 
current (excitation current) to flow in the primary 
winding to set up an alternating magnetic flux in the 
core. The flux cuts across the secondary winding and 
by mutual induction (in practice both windings are 

wound one on the other) a voltage is established in 
the secondary winding. 

When a load is connected to the secondary winding 
terminals, the secondary voltage causes current to 
flow through the winding and a magnetic flux is pro- 
duced which tends to neutralize the magnetic flux 
produced by the primary current. This, in turn, 
reduces the self-induced, or opposition, voltage in 
the primary winding, and allows more current to flow 
in it to restore the core flux to a value which is only 
very slightly less than the no-load value. 

The primary current increases as the secondary 
load current increases, and decreases as the secondary 
load current decreases. When the load is disconnected, 
the primary winding current is again reduced to the 
small excitation current sufficient only to magnetize 
the core. 

To accomplish the function of changing voltage 
from one value to another, one winding is wound with 
more turns than the other. For example, if the prim- 
ary winding has 200 turns and the secondary 1000 
turns, the voltage available at the secondary terminals 

will be -=z-£T, or 5 times as great as the voltage applied 

to the primary winding. This ratio of turns (N 2 ) in the 
secondary to the number of turns (N t ) in the primary 
is called the turns or transformation ratio (r) and it 
is expressed by the equation. 

ffx E l 

where £", and E 7 are the respective voltages of the two 

When the transformation ratio is such that the 
transformer delivers a higher secondary voltage than 
the primary voltage it is said to be of the "step-up" 
type. Conversely, a "step-down" transformer is one 
which lowers the secondary voltage. The circuit arrange- 
ments for both types are also shown in Fig. 4.10. 

Construction of Voltage Transformers. The core of 
a voltage transformer is laminated and conventionally 
is built up of suitably shaped thin stampings, about 
0-012 in. thick on average, of silicon-iron or nickel- 
iron. These materials have the characteristics of fairly 
high resistivity and low hysteresis; therefore, in the 
laminated form, the effects of both eddy currents and 
hysteresis are reduced to a minimum. Two different 
forms of construction are in common use. 

In one the laminations are L-shaped and are 
assembled to provide a single magnetic circuit; in this 


form it is used for the transformation of single-phase 
a.c. The second, known as the shell type, can be used 
for either single-phase or three-phase transformation 
and is one in which half the laminations are U-shaped 
and the remainder are T-shaped, all of them being 
assembled to give a magnetic circuit with two paths. 
In both forms of construction the joints are staggered 
in order to minimize the magnetic leakage at the joints. 
The laminations are held together by core clamps. 

In some designs the cores are formed of strips 
which are wound rather like a clock spring and 
bonded together. The cores are then cut into two C- 
shaped parts to allow the pre-wound coils to be fitted 
The mating surfaces of the two parts are often ground 
to give a very small effective gap which helps to mini- 
mize the excitation current. After assembly of the 
windings the core parts are clamped together by a 
steel band around the outside of the core. 

Transformer windings are of enamelled copper 
wire or strip, and are normally wound on the core 
one upon the other, to obtain maximum mutual 
inductive effect, and are well insulated from each 
other. An exception to this normal arrangement is 
in a variant known as an auto-transformer, in which 
the windings are in series and on a core made up of 
L-shaped laminations. Part of both primary and 
secondary windings are wound on each side of the 
core. On a shell-type transformer both windings are 
wound on the centre limb for single-phase operation, 
and for three-phase operation they are wound on 
each limb. Alternative tappings are generally pro- 
vided on both windings of a transformer for differ- 
ent input and output voltages, while in some types 
a number of different secondary windings provide 
simultaneous outputs at different voltages. 

Circuit Connections. Voltage transformers are 
connected so that the primary windings are in paral- 
lel with the supply voltage; the primary windings of 
current transformers are connected in series. A single- 
phase transformer as the name suggests is for the 
transformation of voltage from a single-phase supply 
or from any one phase of a three-phase supply. Trans- 
formation of three-phase a.c. can be carried out by 
means of three separate single-phase transformers, or 
by a single three-phase transformer. Transformers for 
three-phase circuits can be connected in one of several 
combinations of the star and delta connections (see 
also Chapter 3), depending on the requirements for 
the transformer. The arrangements are illustrated in 
Fig. 4.11. 

When the star connection is used in three-phase 
transformers for the operation of three-phase consumer 
equipment, the transformer may be connected as a 
three-phase system (Fig. 4.1 1(a)). If single-phase loads 
have to be powered from a three-phase supply it is 
sometimes difficult to keep them balanced, it is there- 
fore essential to provide a fourth or neutral wire so 
that connections of the loads may be made between 
this wire and any one of the three-phase lines (Fig. 

3-**iose input 

3-Phose input 

prcwy ■ m i x f mw \ 




Fig 4.11 

Circuit connections for three-phase transformers 

(a) Star connection three-wire 

(b) Star connection four-wire 

(c) Star and Delta connection 

The interconnection of neutral points of two star 
windings is sometimes undesirable because this pro- 
vides an external path for the flow of certain harmonic 
currents which can lead to interference with radio 
communications equipment. This is normally over- 
come by connecting one of the two transformer wind- 
ings in delta, for example, if the transformer supplies 


an unbalanced load, the primary winding is in star 
and the secondary is in delta as shown in Fig. 4.1 1(c). 

Current transformers are used in many a.c. generator 
regulation and protection systems and also in con- 
junction with a.c. ammeters. These transformers have 
an input/output current relationship which is inversely 
proportional to the turns ratio of the primary and 
secondary windings. A typical unit is shown in Fig. 
4.12. It is designed with only a secondary winding on 
a toroidal strip-wound core of silicon-iron. The 
assembly together with the metal base is encapsulated 
in a resin compound moulding. The polarity of the 
transformer is indicated by the markings HI on the 
side facing the generator and H2 on the side facing the 

The primary winding is constituted by passing a 
main cable of the power system, through the core 
aperture. The cable is wound with a single turn if it 
carries high currents, and with two or three turns if it 
carries low currents. The operating principle is the 
same as that of a conventional transformer. 

In some aircraft generating systems, a number of 
current transformers are combined into single package 
assemblies to provide a means of centralizing equip- 
ment location. One such assembly is illustrated in 
Fig. 4.13. It consists of seven transformers which are 
supplied with primary voltage via the three feeder 
terminals and by insulated busbars passing through 
the cores of the transformers which are arranged in 
three sets. The busbars terminate in the flexible 
insulated straps. Secondary leads from the various 

Side HI 

Core and secondary 

Metal base 

transformers are brought out through a common 

Contrary to the practice adopted for voltage trans- 
formers, whenever the secondary windings of current 
transformers are disconnected from their load circuits, 
terminals must be short-circuited together. If this is 
not done, a dangerous voltage may develop which 
may be harmful to anyone accidentally touching the 
terminals, or may even cause an electrical breakdown 
between the windings. 


In circuit applications normally requiring only a small 
step-up or step-down of voltage, a special variant of 
transformer design is employed and this is known as 
an auto-transformer. Its circuit arrangement is shown 
in Fig. 4.14 and from this it will be noted that its 
most notable feature is that it consists of a single 
winding tapped to form primary and secondary parts. 
In the example illustrated the tappings provide a 
stepped-up voltage output, since the number of 
primary turns is less than that of the secondary turns. 
When a voltage is applied to the primary terminals 
current will flow through the portion of the winding 
spanned by these terminals. The magnetic flux due 
to this current will flow through the core and will 
therefore, link with the whole of the winding. Those 
turns between the primary terminals act in the same 
way as the primary winding of a conventional trans- 
former, and so they produce a self-induction voltage 
in opposition to the applied voltage. The voltage 
induced in the remaining turns of the winding will 
be additive, thereby giving a secondary output voltage 

XI X2 

(Start) (Finish) 

O o 

Side HI -towards 



Current- Q carrying conductor 

Identification plate 

Fig 4.12 

Current transformer 


Boost transformers 

Feeder terminal 

Secondary output 

Flexible straps 

Coble harness 

Differential current 

Metering transformer 

Fig 4.13 
Current transformer package 

greater than the applied voltage. When a load circuit 
is connected to the secondary terminals, a current due 
to the induced voltage will flow through the whole 
winding and will be in opposition to the primary 
current from the input terminals. Since the turns 
between the primary terminals are common to input 
and output circuits alike they carry the difference 



Fig 4.14 
Circuit arrangement of an auto-transformer 

between the induced current and primary current, 
and they may therefore be wound with smaller gauge 
wire than the remainder of the winding. 

Auto-transformers may also be designed for use in 
consumer circuits requiring three-phase voltage at 
varying levels. The circuit arrangement of a typical 
step-up transformer applied to a windshield anti- 
icing circuit is shown in Fig. 4.1 5. The three windings 
are star-connected and are supplied with the "primary" 
voltage of 208 volts from the alternator system. The 
secondary tappings are so arranged that up to four 
output voltage levels may be utilized. 

Transformers are usually rated in volt-amperes or 
kilovolt-amperes. The difference between the output 
terminal voltages at full-load and no-load, with a con- 
stant input voltage, is called the regulation of the 
transformer. As in the case of an a.c. generator, 
regulation is expressed as a percentage of the full-load 
voltage, and depends not only on actual losses (e.g. 
hysteresis, eddy current and magnetic leakage) but 
also on the power factor of the load. Thus, an 
inductive load, i.e. one having a lagging power factor, 
will give rise to a high percentage regulation, while 
with a capacitive load, i.e. one having a leading power 


270 -V Phose 
410 -V output 

0-V Phose'B' Jo 
0-V output \o 


270 -V Phose 
410-V output 


Phose A 

Phose B 

Input 208 -V 
Output 480 -V 
Output 320-V 

Input 208 -V 
Output 480-V 
Output 320-V 

[ Input 208 -V 

L Phose'C' 1 Output 480-V 

[Output 320-V 

Fig 4.15 

Tappings of a typical three-phase auto-transformer 

factor, the regulation may be a negative quality giving 
a higher output voltage on full-load than on no-load. 

Changes in power supply frequency, or the con- 
nection of a transformer to a supply whose frequency 
differs from that for which the transformer was 
designed, has a noticeable effect on its operation. This 
is due to the fact that the resistance of primary wind- 
ings are so low that they may be considered to be a 
purely inductive circuit. If, for example, the frequency 
is reduced at a constant value of voltage, then the 
current will rise. The increased current will, in turn, 
bring the transformer core nearer to magnetic satura- 
tion and this decreases the effective value of inductance 
leading to still larger current. Thus, if a transformer 
is used at a frequency lower than that for which it 
was designed, there is a risk of excessive heat genera- 
tion in the primary winding and subsequent burn out. 
On the other hand, a transformer designed for low 
frequency can be used with higher frequencies, since 
in this case the primary current will be reduced. 

Transformer-rectifier units (T.R.U.) are combina- 
tions of static transformers and rectifiers, and are 
utili7.ed in some a.c. systems as secondary supply 
units, and also as the main conversion units in aircraft 
having rectified a.c. power systems. 

Fig. 4.16 illustrates a T.R.U. designed to operate 
on a regulated three-phase input of 200 volts at a 
frequency of 400 Hz and to provide a continuous 
d.c. output of 1 1 A at approximately 26 volts. The 
circuit is shown schematically in Fig. 4.17. The unit 
consists of a transformer and two three-phase bridge 
rectifier assemblies mounted in separate sections of 
the casing. The transformer has a conventional star- 
wound primary winding and secondary windings 
wound in star and delta. Each secondary winding is 
connected to individual bridge rectifier assemblies 
made up of six silicon diodes, and connected in 
parallel. An ammeter shunt (dropping 50 mV at 
100 A) is connected in the output side of the rect- 
ifiers to enable current taken from the main d.c. 
output terminals to be measured at ammeter auxiliary 
terminals. These terminals, together with all others 
associated with input and output circuits, are grouped 
on a panel at one end of the unit. Cooling of the unit 
is by natural convection through gauze-covered ven- 
tilation panels and in order to give warning of over- 
heating conditions, thermal switches are provided at 
the transformer and rectifier assemblies, and are 
connected to independent warning lights. The switches 
are supplied with d.c. from an external source 
(normally one of the busbars) and their contacts 
close when temperature conditions at their respective 
locations rise to approximately 150°C and 200° C. 

Transformer section 

A.C. input 


Temperature warning 
system terminals 

DC. output 

Fig 4.16 
Transformer-rectifier unit 


Schematic circuit of a transformer-rectifier unit 

o To warning 

o +D.C input 

To ammeter 

Rotary Converting Equipment 

The most commonly used item to be included under 
this heading is the machine which converts d.c. into 
a.c. and is variously called a "rotary converter", 
"motor-generator" and an "inverter". All three 
terms can, understandably, cause some confusion 
regarding their definition, with the result that they 
tend to be loosely applied to machines which, 
although performing the same function, have quite 
different constructional and electrical circuit features. 
It is not the intention here to justify terminology and 
applications but the following details may serve to 
clarify the position. 

Rotary Converter. This is, by definition, "a synchro- 
nous machine with a single armature winding having 
a commutator and slip-rings for converting a.c. into 
d.c. or vice versa" (B.S.4727). These machines are 
not used in aircraft and where the term "rotary con- 
verter" is applied reference to an inverter is more 
often than not intended. 

Motor-generators. These are a "combination of one 
or more generators directly coupled to one or more 
motors" (B.S.4727); thus a unit essentially comprises 
two electrically separate machines mechanically 
coupled. A d.c. to a.c. type of unit is employed in 
one or two types of aircraft for the supply of 
secondary a.c. power, and in such an application is 

sometimes referred to as a motor alternator and also 
as an "inverter". 

Inverter. This term is generally accepted as referring 
to a d.c. to a.c. type of rotary converter having separ- 
ate d.c. armature and a.c. rotor windings, located in 
the same slots and sharing the same field system. The 
a.c. output is derived from the rotor via slip-rings. 


Fig. 4. 1 8 shows a type of motor-generator which is 

Voltage and frequency 
control unit 

Moto' section 
Shock mounting 

Fig 4.18 

Motor generator 


designated as an inverter by the particular manu- 
facturer. The motor is a four-pole compound-wound 
machine supplied from a 28-volts d.c. source, and 
driving a three-phase 1 1 5-volts a.c. generator through 
a common shaft. Cooling is by means of fans one at 
each end of the machine. Direct current is supplied 
to the motor via its brush-gear, and also to the gener- 
ator rotor field windings via brushes and slip-rings. 
The voltage and frequency of the generator output 
are regulated by independent carbon pile regulators 
which form part of a control panel mounted above 
the motor and generator. The unit is supported in 
an anti-vibration mounting which is secured to the 
appropriate part of the aircraft's structure. Operating 
principles will be described later in this chapter. 
Fig. 4.19 is a sectional view of an "inverter", 
which performs the same function as the motor- 
generator just described, but by means of a common 
armature and field system. The d.c. section of the 
machine is also of the four-pole compound-wound 
type, the d.c. being supplied to the armature winding, 

Field coil 

AC brushgear 


DC brushgear 

Fig 4.19 


series and shunt-field windings by means of the 
commutator and brush-gear. The a.c. section corre- 
sponds to a star-wound generator, the winding being 
located in the slots of the armature and beneath the 
d.c. winding. The a.c. winding is connected to a 
triple slip-ring and brush-gear assembly at the opposite 
end to the commutator. Thus, when the inverter is 
in operation, a three-phase output is induced in a 
rotating winding and not a fixed stator winding as in 
the case of a conventional a.c. generator. The output 
voltage and frequency are regulated by a separate 

control panel. Cooling of the inverter is by means of 
a fan fitted at the slip-ring end of the armature shaft. 

The control of the output voltage and frequency 
between close limits is a very important aspect of 
rotary converting equipment operation, and the con- 
trol methods adopted are varied but, in most cases, 
are based on those utilized in the control systems of 
d.c. and a.c. generators. The operating principles of 
some typical control systems are described in the 
following paragraphs. 

Fig. 4.20 shows in simplified form the circuit 
of a control system designed for the regulation of 
voltage and frequency of the motor-generator illus- 
trated in Fig. 4.18, and serves as a further example 
of the application of carbon-pile regulator principles 
(see Chapter 1 p. 11). 

When the machine is switched on the starting relay 
is energized and direct current is fed, via the relay 
contacts, to the field system of the motor and also to 
the field winding on the generator rotor. Thus, both 
fields are excited, and the rotor drives the generator 
which produces a three-phase output in its stator 
winding. As in the case of a d.c. generator system, the 
carbon piles are series connected to the field windings, 
the one in the motor circuit controlling the frequency 
by virtue of the fact that its resistance controls the 
speed of the motor and generator. The voltage coil of 
each regulator is supplied with d.c. from selenium 
rectifiers connected to one phase of the generator 

If during operation, the generator output voltage 
should fluctuate then an increased or decreased 
current will flow through the coil of the voltage 
regulator to vary the pile resistance and generator 
field excitation, to restore the voltage to its nominal 
value. Similarly, if the frequency of the output should 
fluctuate, an increased or decreased current will flow 
through the frequency regulator coil. The effect is 
obtained by means of a condensor and an inductor 
combined to form a parallel tuned circuit in which 
the frequency determines the amount of current 
passed. In this case, coil current diminishes as the 
frequency rises and increases as the frequency drops. 
Since the frequency regulator pile is in series with 
the shunt field of the motor, a change in pile resist- 
ance acts to regulate frequency by controlling the 
motor and generator speed. Weakening of the shunt 
field will cause the motor to speed up and vice versa. 

If variation of the d.c. supply to the motor should 


Fig 4.20 

Carbon-pile type voltage and frequency control system 

28V bus-bor 

28 Vdc. supply 

1 15 V output 

Rectified oc. 

(voltoge and frequency control) 





Fig 4.21 

Inverter voltage and frequency control system 


occur, the increased or decreased motor speed will 
result in both a voltage and frequency fluctuation in 
the generator output so that both regulators will 
come into operation together to correct the variation. 

Fig. 4.21 shows the controlling circuit of the 
machine illustrated in Fig. 4.19, and as will be noted 
it is a straightforward application of the carbon pile 
regulator principle. The a.c. output is rectified and 
supplied to the voltage coil of the regulator which 
varies the pile resistance in the usual manner, this, 
in turn, varying the current flow through the common 
field system to keep both the voltage and frequency 
of the a.c. output within limits. 


These inverters perform the same conversion function 
as the rotary machines described earlier, but by means 
of solid-state or static circuit principles. They are 
employed in a number of types of aircraft in some 
cases as a normal source of a.c. power, but more 
usually to provide only emergency a.c. power to 
certain essential systems when a failure of the normal 
1 1 5-volts source has occurred. The function of an 
inverter used for the conversion of battery supply to 
single-phase 1 1 5-volts a.c. is shown in the block 
diagram of Fig. 4.22. 

The d.c. is supplied to transistorized circuits of a 
filter network, a pulse shaper, a constant current 
generator, power driver stage and the output stage. 
After any variations in the input have been filtered 
or smoothed out, d.c. is supplied to a square-wave 
generator which provides first-stage conversion of the 
d.c. into square-wave form a.c. and also establishes the 
required operating frequency of 400 Hz. This output is 
then supplied to a pulse shaper circuit which controls 
the pulse width of the signal and changes its wave 
form before it is passed on to the power driver stage. 
It will be noted from the diagram that the d.c. required 
for pulse shaper operation is supplied via a turn-on 
delay circuit. The reason for this is to cause the pulse 
shaper to delay its output to the power driver stage 
until the voltage has stabilized. The power driver 
supplies a pulse-width modulated symmetrical output 
to control the output stage, the signal having a square- 
wave form. The power driver also shorts itself out each 
time the voltage falls to zero, i.e. during "notch time". 

The output stage also produces a square-wave out- 
put but of variable pulse width. This output is finally 
fed to a filter circuit which reduces the total odd 
harmonics to produce a sine wave output at the volt- 

age and frequency required for operating the systems 
connected to the inverter. 

As in the case of other types of generators, the 
output of a static inverter must also be maintained 
within certain limits. In the example illustrated, this 
is done by means of a voltage sensor and a current 
sensor, both of which produce a rectified a.c. feed- 
back signal which controls the "notch time" of the 
pulse shaper output through the medium of a regula- 
tor circuit and a notch control circuit. 

Test Questions 

1. Rectification is the process of converting: 

(a) a high value of a.c. into a lower value. 

(b) d.c. into a.c. 

(c) a.c. into d.c. 

2. Describe the fundamental principle on which 
rectification is based. 

3. An "n-type" semiconductor element is one 

(a) an excess of "holes". 

(b) a deficiency of "holes". 

(c) an excess of electrons. 

4. What semi-conductor elements are usually employe 
in rectifiers used in aircraft? Describe the con- 
struction of one of these rectifiers. 

5. What is meant by the term Zener voltage? 

6. Is the Zener voltage of any practical value in recti- 
fication equipment? 

7. Explain the operating principle of a silicon- 
controlled rectifier (S.C.R.). With a suitable dia- 
gram, show a practical example of the use of an 
S.C.R. in modern aircraft practice. 


8. With the aid of a circuit diagram explain how full- 
wave rectification of a three-phase input takes 

9. Describe the basic construction and principle of 
the device used for converting alternating current 
from one value to another. 

1 0. What is meant by transformation ratio and how 
is it applied to "step-up" and "step-down" trans- 

11. Draw a circuit diagram to illustrate a star- 
connected three-phase transformer. 

12. Describe the operation of a current transformer. 
For what purpose is such a device used? 

13. What effects do changes in frequency have on the 
operation of a transformer? 

* s 
.9 is 

u. > 


14. With the aid of a circuit diagram, describe the 
operating principle of a typical transformer- 
rectifier unit. 

15. For what purpose are the power converting 
machines of the rotary type utilized in aircraft? 

16. Describe how the carbon-pile principle is applied 
to the regulation of the frequency of an inverter 

17. Describe how transistors are utilized for the con- 
version of electrical power supplies and regulation 
of voltage and frequency levels. 

1 8. With the aid of sketches, explain how the voltage 
and frequency output are maintained constant on 
a rotary inverter which converts 28-volts d.c. to 

1 1 5-volts a.c. 



Ground Power Supplies 

Electrical power is required for the starting of engines, 
operation of certain services during "turn-round" 
servicing periods at airports, e.g. lighting, and for the 
testing of electrical systems during routine main- 
tenance checks. The batteries of an aircraft are, of 
course, a means of supplying the necessary power, and 
although capable of effecting engine starts their 
capacity does not permit widescale use on the ground 
and as we have already learned from Chapter 2, they 
are restricted to the supply of power under emergency 
conditions. It is necessary, therefore, to incorporate a 
separate circuit through which power from an external 
ground power unit (see Fig. 5.1) may be connected to 
the aircraft's distribution busbar system. In its sim- 
plest form, a ground power supply system consists of 
a connector located in the aircraft at a conveniently 
accessible point (at the side of a fuselage for example) 
and a switch for completing the circuit between the 
ground power unit and the busbar system. 

In addition to the ground power supply system, 
some types of aircraft carry separate batteries which 

Control panels 

'owet receptacle 

Fig 5.1 

Ground power unit 

can supply the ground services in the event that a 
ground power unit is not available in order to conserve 
the main batteries for engine starting. 

D.C. Systems 

A basic system for the supply of d.c. is shown in Fig. 
5.2, and from this it will also be noted how, in addi- 
tion to the ground power supply, the battery may be 
connected to the main busbar by selecting the 
"flight" position of the switch. As the name suggests 
this is the position to which the switch is selected when 
the aircraft is in flight since under this condition the 
generator system supplies the main busbar and the 
battery is constantly supplied with charging current. 

Moin dx. bus-bar 

Battery bus-bor 


| Power selector 
■ swi'ch 





Groona power 

Fig 5.2 

Basic ground power supply system 


The ground power connector symbol shown in the 
diagram represents a twin-socket type of unit which is 
of an early design and now limited in its application. 
The sockets and corresponding plug pins are of dif- 
ferent diameters to prevent a reverse polarity condi- 
tion, and, in order to make the connection, the outer 
cover of the unit must be rotated to expose the 
sockets. As a result of standardization requirements, 
multi-pin plug units were introduced and are now 
employed in nearly all types of aircraft. A typical 
three-pin plug, shown in Fig. 5.3, consists of two 
positive pins and one negative pin; one of the positive 
pins is shorter and of smaller diameter than the re- 
maining pins. The pins are enclosed by a protective 
shroud, and the complete unit is normally fitted in a 
recessed housing located at the appropriate part of 
the airframe structure. Access to the plug from outside 
the aircraft, is via a hinged flap provided with quick- 
release fasteners. 

3- pin plug 

Moin dc. bus-bar 

External supply 

Access door 

Fig 5 J 

Ground power supply plug 

A circuit employing a multi-pin plug unit is illus- 
trated in Fig. 5.4, and from this it will be noted that 
the short positive pin is connected in the coil circuit 
of the ground power relay. The reason for this is that 
in the event of the ground supply socket being with- 
drawn with the circuit "live", the ground power relay 
will de-energize before the main pins are disengaged 
from the socket. This ensures that breaking of the 
supply takes place at the heavy-duty contacts of the 
relay thus preventing arcing at the main pins. 

In some aircraft d.c. power is distributed from a 
multiple busbar system and it is necessary for certain 
services connected to each of the busbars to be oper- 
ated when the aircraft is on the ground. This requires 
a more sophisticated arrangement of the ground power 
supply system and the circuit of one such arrangement 

Reverse currenl 
circuit breoker 


Ground power reloy 





1 Power 
| selector 
' switch 




Ground power 

Fig 5.4 

Multi-pin power plug system 

is shown in Fig. 5.5. In addition to the ground supply 
relay or contactor, contactors for "tying" busbars 
together are provided, together with magnetic indica- 
tors to indicate that all connections are made. 

When the ground power supply unit is connected 
to the aircraft and the master switch is selected "on", 
it energizes the ground supply contactor, thus closing 
its auxiliary and main sets of contacts. One set of 
auxiliary contacts complete a circuit to a magnetic 
indicator which then indicates that the ground supply 
is connected and on ("C" in Fig. 5.5), a second set 
complete circuits to the coils of No. 1 and No. 3 bus- 
tie contactors while a third and main heavy-duty set 
connect the supply direct to the "vital" and No. 2. 
busbars. When both bus-tie contactors are energized 
their main contacts connect the supply from the 
ground supply contactor to their respective busbars. 
Indication that both busbars are also "tied" to the 
ground power supply is provided by magnetic indica- 
tors "A" and "B" which are energized from the vital 
busbar via the auxiliary contacts of the contactor. 

A.C. Systems 

In aircraft which from the point of view of electrical 
power are principally of the "a.c. type", then it is 
essential for the ground supply system of the intall- 
ation to include a section through which an external 


Ground Ofi 
supply | 
maste r i 
switch On 


No.l di.bus Vital debus No.2 debus No.3d.c.bus 

No. I bus tie 



i-^M ' I : J — ' 

Cfc . \-(MCir Aux. 

Aux. >£ i"D 

*=*_*- »— H.i.J- — 


Ground power plug 

No. 3 bus tie 






-J Main 

Fig 5.5 

Schematic of a ground power supply - multiple d.c. busbar 

source of a.c. power may be supplied. The circuit 
arrangements for the appropriate systems vary be- 
tween aircraft types but in order to gain some under- 
standing of the circuit requirements and operation 
generally we may consider the circuit shown in Fig. 

When ground power is coupled to the plug a three- 
phase supply is fed to the main contacts of the ground 
power breaker, to a ground power transformer/ 
rectifier unit (T.R.U.) and to a phase sequence pro- 
tection unit. The T.R.U. provides a 28 volt d.c. feed- 
back supply to a hold-in circuit of the ground power 
unit. If the phase sequence is correct the protection 
unit completes a circuit to the control relay coil, thus 
energizing it. A single-phase supply is also fed to an 
amber light which comes on to indicate that ground 
power is coupled, and to a voltmeter and frequency 
meter via a selector switch. 

The circuit is controlled by a ground power switch 
connected to a busbar supplied with 28 volts d.c. from 
the aircraft battery system. When the switch is set to 

the "close" position current flows across the main 
contacts of the energized control relay, to the "close" 
coil of the ground power breaker, thus energizing it to 
connect the ground supply to the three-phase a.c. 
main busbar. The ground power supply is disconnected 
by selecting the "trip" position on the ground power 
switch. This action connects a d.c. supply to the trip 
coil of the ground power breaker, thus releasing its 
main and auxiliary contacts and isolating the ground 
power from the a.c. main busbar. 


Many of today's aircraft are designed so that if neces- 
sary, they may be independent of ground support 
equipment. This is achieved by the incorporation of 
an auxiliary power unit (A.P.U.) which, after being 
started by the aircraft's battery system, provides 
power for engine starting, ground air conditioning 
and other electrical services. In some installations, 
the A.P.U. is also used for supplying power in flight 
in the event of an engine-driven generator failure and 

r- i 




for supplementing the delivery of air to the cabin 
during take-off and climb. 

In general, an A.P.U. consists of a small gas turbine 
engine, a bleed-air control and supply system, and an 
accessory gearbox. The gas turbine comprises a two- 
stage centrifugal compressor connected to a single- 
stage turbine. The bleed-air control and supply sys- 
tem automatically regulates the amount of air bleed 
from the compressor for delivery to the cabin air 
conditioning system. In addition to those accessories 
essential for engine operation, e.g. fuel pump control 
unit and oil pumps, the accessory gearbox drives a 
generator which, depending on the type required for 
a specific aircraft, may supply either d.c. or a.c. 

A motor for starting the A.P.U. is also secured to 
the gearbox and is operated by the aircraft battery 
system or, when available, from a ground power unit. 
In some types of A.P.U. the functions of engine 
starting and power generation are combined in a 
starter/generator unit. 

An external view of a typical unit and location in 
an aircraft is shown in Fig. 5.7. 

Test Questions 

1. Explain why it is necessary for a ground power 
supply circuit to form part of an aircraft's electrical 

2. Draw a diagram of a basic d.c. power supply circuit 
and explain its operation. 

3. In a multi-pin plug how is it ensured that the breaking 
of the ground power supply circuit takes place with- 
out arching? 

4. Draw a diagram of a ground power supply circuit 
of a typical "all-a.c." aircraft and explain its 

5. (a) State the purpose of an A.P.U. fitted on a 

modern aircraft. 

(b) What services are usually provided by the 

(c) Detail the safety devices which are necessary in 
the complete installation. 



Measuring Instruments, 
Warning Indicators and Lights 

In order to monitor the operating conditions of the 
various supply and utilization systems, it is necessary 
for measuring instruments and warning devices, in the 
form of indicators and lights, to be included in the 
systems. The number of indicating devices required 
and the types employed depend on the type of air- 
craft and the overall nature of its electrical installation. 
However, the layout shown in Fig. 6.1 is generally 
representative of systems monitoring requirements 
and can usefully serve as a basis for study of the ap- 
propriate indicating devices. 

These instruments are provided in d.c. and a.c. power 
generating systems and in most instances are of the 
permanent magnet moving-coil type shown in basic 
form in fig. 6.2. 

An instrument consists essentially of a permanent 
magnet with soft-iron pole pieces, between which a 
soft-iron core is mounted. A coil made up of a num- 
ber of turns of fine copper wire is wound on an 
aluminium former which in turn is mounted on a 
spindle so that it can rotate in the air gap between the 
pole pieces and the core. The magnetic field in the 
air gap is an intense uniform radial field established 
by the cylindrical shape of the pole pieces and core. 
Current is led into and out of the coil through two 
hairsprings which also provide the controlling force. 
The hairsprings are so mounted that as the coil rotates, 
one spring is unwound and the other is wound. A 
pointer is attached to the spindle on which the moving 
coil is mounted. 

When current flows through the coil a magnetic 
field is set up which interacts with the main field in 
the air gap such that it is strengthened and weakened 
as shown in the diagram. A force (Fd) is exerted on 
each side of the coil, and the couple so produced 
causes the coil to be rotated until it is balanced by 

the opposing controlling force (Fc) of the hairsprings. 
Thus, rotation of the coil and pointer to the equili- 
brium position is proportional to the current flowing 
through the coil at that instant. This proportionality 
results in the evenly divided scale which is a charac- 
teristic of the moving coil type of indicator. When the 
coil former rotates in the main field, eddy currents 
are induced in the metal and these react with the main 
field producing a force opposing the rotation, thus 
bringing the coil to rest with a minimum of oscillation. 
Indicators of this kind are said to be "dead beat". 

In order to protect the movements of these instru- 
ments against the effects of external magnetic fields 
and also to prevent "magnetic leakage", the move- 
ments are enclosed in a soft-iron case which acts as a 
magnetic screen. The soft-iron has a similar effect to 
the core of the indicator, i.e. it draws in lines of force 
and concentrates the field within itself. 

Moving coil instruments are also generally employed 
for the measurement of voltage and current in an a.c. 
system. Additional components are necessary, of 
course, for each measuring application; e.g. for the 
measurement of voltage, the instrument must also 
contain a bridge rectifier while for the measurement 
of current, a shunt and a transformer are required in 
addition to the bridge rectifier. 

Reference to Fig. 6.1 shows that all the instru- 
ments located on the control panel are of the circular- 
scale type; a presentation which is now adopted in 
many current types of aircraft. It has a number of 
advantages over the more conventional arc-type scale; 
namely, that the scale length is increased and for a 
given measuring range, the graduation of the scale can 
be more open, thus helping to improve the observa- 
tional accuracy. 

In order to cater for this type of presentation, it is, 
of course, necessary for some changes to be made in 
the arrangement of the magnet and moving coil 



r i ac 

ira oc 

Fig 6.1 

Electrical system control panels 

systems, and one such arrangement is illustrated in 
Fig. 6.3. 

The magnet is in the form of a block secured to a 
pole piece which is bored out to accommodate a core 
which itself is slotted and bored to permit the posi- 
tioning of the moving coil. The coil former, unlike 
that of a conventional instrument, is mounted to one 
side of its supporting spindle, and under power-off 
conditions it surrounds the core and lies in the air 
gap at the position shown. The field flows from the 
magnet to the core which, in reality, forms a North 

pole, and then across the air gap to the pole piece 
forming the South pole. The return path of the field 
to the South pole of the magnet is completed through 
the yoke, which also shields the flux from distortion 
by external magnetic fields. When current flows 
through the coil, a force is produced due to the inter- 
action between the permanent magnetic field and the 
induced field, but unlike the conventional instrument 
the coil is rotated about the core by a force acting on 
one side only; the opposite side being screened from 
the flux by the core itself. 

Zero oojus'er 

Pointer ond 
'balancing a*ms 

Pole p<eces 

field due lo 


Fig 6.2 

Basic form of moving coil indicator 

Magnet ond 
magnet spacer 

Fig 6.3 

Magnet system of a typical long-scale moving coil instrument 



Shunts are used in conjunction with all d.c. system 
ammeters, and where specified, in a.c. systems, and 
their main purpose is to permit an ammeter to measure 
a large number of possible values of current, i.e. they 
act as range extension devices. Fundamentally, a shunt 
is a resistor having a very low value of resistance and 
connected external to the ammeter and in parallel 
with its moving coil. The materials used for shunts are 
copper, nichrome, manganin, minalpha and telcumen. 
Typical shunts used in d.c. and a.c. generating sys- 
tems are illustrated in Fig. 6.4 and although their 
principal physical features differ, a feature common 
to all shunts should be noted and that is they are 

Current (mom) lerminols 

Potential (ammeter) 

Current (main) 

Potential (ommeter} 

Potential lommeter] 

Fig 6.4 

each provided with four terminals. Two of these are 
of large current-carrying capacity ("current" ter- 
minals) for connecting the shunt in series with the 
main circuit, and two are of smaller size to carry 
smaller current ("potential" terminals) when con- 
nected to the associated ammeter. The unit shown at 
(a) employs strips of lacquered minalpha spaced from 
each other to promote a good circulation of air and 
thus ensure efficient cooling. 

When the ammeter is in series with the main cir- 
cuit only a fraction of the current passes through the 
moving coil, the remainder passing through the shunt 
which is selected to carry the appropriate load with- 
out overheating. The scale of the ammeter is, however, 
calibrated to indicate the range of current flow in the 
main circuit, since the flow through the coil and the 
shunt are in some pre-calculated ratio. 

Transformers are used in conjunction with a.c. 
measuring instruments, and they perform a similar 
function to shunts, i.e. they permit a "scaling-down" 
of large currents and voltages to a level suitable for 
handling by standardized types of instruments. They 
fall into two main classes: (i) current or series 
transformers and (ii) potential or parallel trans- 
formers. The construction and operation of both 
classes has already been dealt with in Chapter 4 and 
at this stage therefore we shall only concern ourselves 
with typical applications. 

Current transformers are normally used with a.c. 
ammeters and Fig. 6.5 illustrates a typical circuit 
arrangement. The main current-carrying conductor 
passes through the aperture of the secondary windings, 
the output of which is supplied to the ammeter via 
a bridge rectifier, which may be a separate unit or 
form part of the instrument itself. 

An applicaiton of a potential transformer is illus- 
trated in Fig. 6.6 and it will be noted that in this case 
the transformer forms part of a shunt, the primary 
winding being connected to the current terminals 1 
and 4. The voltage developed across the shunt is 
stepped-up in the transformer to a maximum r.m.s. 
value (2-5 volts in this particular example) when 
the rated current is flowing through the shunt. The 
transformer output is connected to the "potential" 
terminals 2 and 3 and is rectified within the relevant 
ammeter and then applied to the moving coil. The 
scale of the ammeter used with this transformer 
arrangement is non-linear because the deflection of 
the moving coil is not proportional to the current 




a ,00000110000, - 

Main cu 


rrent- jf carryingi 

Current transformer 

Fig 6.5 

Application of a current transformer 


transformer j 

! " — ^smssu — ' 




Ma»n current- 
carrying conductor 



Fig 6.6 

Application of a potential transformer 

flowing through the shunt as a result of the sum of 
non-linear characteristics of the transformer and 

Fig. 6.7 illustrates a circuit arrangement adopted 
for the measurement of d.c. loads in a rectified a.c. 
power supply system. The ammeter is utilized in con- 
junction with a three-phase current transformer, bridge 
rectifier and a shunt, which form an integrated unit of 



39 vor. 





- JUJUSv- 1 

— - -/-lilt 

Fig 6.7 

Measurement of d.c. loads in a rectified a.c. system 

the type shown in Fig. 6.8, and also a main shunt 
similar to that employed in basic d.c. generating sys- 
tems. The ammeter is calibrated in amperes d.c. and 
it may be connected into either one of two circuits 
by means of a selector switch marked "D.C." and 
" A.C.". In the "D.C." position the ammeter is selected 
in parallel with the main shunt so that it measures the 
total rectified load taken from the main d.c. busbar. 

When the "A.C." position is selected, the ammeter 
is connected to the shunt of the current transformer 
unit and as will be noted from the circuit diagram, this 
unit taps the generator output lines at a point before 
the main d.c. output rectifier. The transformer output 
is rectified for measuring purposes, so therefore in the 
"A.C." position of the switch, the ammeter will 
measure the d.c. equivalent of the total unrectified 


Fig 6.8 

Three-phase current transformer unit 


These instruments form part of the metering system 
required for main a.c. power generating systems, and 
in some aircraft, they may also be employed in 
secondary a.c. generating systems utilizing inverters. 
The dial presentation and circuit diagram of a typical 
meter are shown in Fig. 6.9. The indicating 

element, which is used in a mutual inductance circuit, 
is of the standard electrodynamometer pattern con- 
sisting essentially of a moving coil and a fixed field 
coil. The inductor circuit includes a nickel-iron core 
loading inductance, a dual fixed capacitor unit, four 
current-limiting resistors connected in series-parallel, 
and two other parallel-connected resistors which pro- 
vide for temperature compensation. The electrical 
values of all the inductor circuit components are 

The instrument also incorporates a circuit which 
is used for the initial calibration of the scale. The 
circuit is comprised of a resistor, used to govern the 
total length of the arc over which the pointer travels 
between the minimum and maximum frequencies, 
and a variable inductor system which governs the 
position of the centre of the arc of pointer travel 
relative to the mid-point of the instrument scale. 

In operation the potential determined by the 
supply voltage and frequency is impressed on the field 
coil, which in turn sets up a main magnetic field in 
the area occupied by the moving coil. A second poten- 


o — 









ea CI 

018 ^F - 0033 M F 

3-2 kXi(ol37-8°C) 
negative temperoture 

Temperoture compensator 

2-5 kft 


in test 

Upper i Lower 

control (TO) rtj J control 
spring ] S spring 

KS&QQ./ Moving coil 


Field coil 

Current limiting resistors 

Fig 6.9 

Circuit arrangements of a frequency meter 


tial, whose value is also dependent on the supply 
voltage and frequency, is impressed on the moving 
coil, via the controlling springs. Thus, a second mag- 
netic field is produced which interacts with the main 
magnetic field and also produces a torque causing the 
moving coil to rotate in the same manner as a con- 
ventional moving coil indicator. Rotation of the coil 
continues until the voltage produced in this winding 
by the main field is equal and opposite to the im- 
pressed potential at the given frequency. The total 
current in the moving coil and the resulting torque are 
therefore reduced to zero and the coil and pointer 
remain stationary at the point on the scale which 
corresponds to the frequency impressed on the two 


In some a.c. power generating systems it is usual to 
provide an indication of the total power generated 
and/or the total reactive power. Separate instruments 
may be employed; one calibrated to read directly in 
watts and the other calibrated to read in var's (volt- 
amperes reactive) or, as in the case of the instru- 
ment illustrated in Fig. 6.10, both functions may be 
combined in what is termed a watt/var meter. 

The construction and operation of the meter, not 
unlike the frequency meter described earlier, is based 
on the conventional electrodynamometer pattern and its 
scale, which is common to both units of measurement, 
is calibrated for use with a current transformer and an 
external resistor. A selector switch mounted adjacent 
to the meter provides for it to be operated as either 
a wattmeter or as a varmeter. 

When selected to read in watts the field coil is 
supplied from the current transformer which as will 
be noted from Fig. 6.10 senses the load conditions 
at phase "B" of the supply. The magnetic field pro- 
duced around the field coil is proportional to the load. 
The moving coil is supplied at 1 1 5 volts from phase B 
to ground and this field is constant under all condi- 
tions. The currents in both coils are in phase with 
each other and the torque resulting from both mag- 
netic fields deflects the moving coil and pointer until 
balance between it and controlling spring torque is 

In the "var" position of the selector switch the 
field coil is again supplied from the current transformer 
sensing load conditions at phase "B". The moving coil, 
however, is now connected across phases "A" and 
"C" and in order to obtain the correct coil current, a 
calibrated resistor is connected in the circuit and 

j Externol 
. resistor 

Fig 6.10 
Circuit arrangements of a watt/VAR meter 

mounted external to the instrument. The current in 
the moving coil is then at 90 degrees to the field coil 
current, and if the generator is loaded at unity power 
factor, then the magnetic fields of both coils bear the 
same angular relationship and no torque is produced. 

For power factors less than unity there is inter- 
action of the coil fields and a torque proportional to 
the load current and phase angle error is produced. 
Thus, the moving coil and pointer are rotated to a 
balanced position at which the reactive power is indi- 

Warning and indicator lights are used to alert the 
flight crew to conditions affecting the operation of 
aircraft systems. The lights may be divided into dif- 
ferent categories according to the function they per- 
form, and in general, we find that they fall into three 
main categories: (i) warning lights, (ii) caution lights 
and (iii) indicating or advisory lights. 

Warning Lights. These are designed to alert the 
flight crew of unsafe conditions and are accordingly 
coloured red. 

Caution Lights. These are amber in colour to indi- 
cate abnormal but not necessarily dangerous condi- 


tions requiring caution, e.g. hydraulic system pressure 
running low. 

Indicating or Advisory Lights. These lights, which are 
either green or blue, are provided to indicate that a 
system is operable or has assumed a safe condition, 
e.g. a landing gear down and locked. 

Warning and indicator light assemblies are, basically, 
of simple construction, consisting of a bulb contained 
within a casing which incorporates electrical contacts 
and terminals for connection into the appropriate cir- 
cuit. The coloured lens is contained within a cap 
which fits over the casing and bulb. Provision for 
testing the bulb to ensure that its filament is intact 
is also incorporated in many types of light assemblies. 
The lens cap is so mounted on the casing, that it can 
be pressed in to connect the bulb directly to the main 
power supply. Such an arrangement is referred to as 
a "press-to-test" facility. 

Lights may also include a facility for dimming and 
usually this may be done in either of two ways. A 
dimming resistor may be included in the light circuit, 
or the lens cap may incorporate an iris type diaphragm 
which can be opened or closed by rotating the cap. 
Lights used for warning purposes do not usually in- 
clude the dimming facility because of the danger 
involved in having a dimmed warning light escaping 

The power supplies for warning and indicator 
lights are derived from the d.c. distribution system 
and the choice of busbar for their connection must 
be properly selected. For example, if the failure of a 
system or a component is caused by the loss of supply 
to an auxiliary busbar, then it is obvious that if the 
warning light system is fed from the same busbar 
warning indications will also be lost. To avoid this 
risk it is necessary for warning lights to be supplied 
from busbars different from those feeding the assoc- 
iated service, and preferably on or as close as possible 
electrically to the busbar. Caution and indicating 
lights may also, in some cases, be supplied in a similar 
manner, but usually they are supplied from the same 
busbar as the associated service. 


In certain types of aircraft utilizing constant-frequency 
a.c. generating systems, lights are provided to indicate 
synchronism between generator output voltage and 
are used when manually switching in a generator to 
run in parallel with others. A typical four-generator 
system has two lights; one connected between phase 

"A" of the generator it is required to select and phase 
"A" of a synchronizing busbar, and the other light 
between phase "C" of the same units. If the generator 
selected is in synchronism with the other generator 
supplying the synchronizing busbar, the voltages at 
phases "A" and "C" of all generators will have the 
same magnitude and polarity at that instant. The 
lights will therefore be extinguished and the genera- 
tor may be switched on. If, on the other hand, a slight 
phase or frequency difference exists, the lights will be 
illuminated continuously or will flash on and off. 
Under these conditions the generator must remain 
switched off in order to prevent shock loads from 
being imposed on the drive shaft. 


In many types of aircraft system, components require 
electrical control; for example, in a fuel system, 
electric actuators position valves which permit the 
supply of fuel from the main tanks to the engines and 
also for cross-feeding the fuel-supply. All such devices 
are, in the majority of cases, controlled by switches 
on the appropriate systems panel, and to confirm the 
completion of movement of the device an indicating 
system is necessary. 

The indicating system can either be in the form of 
a scale and pointer type of instrument, or an indicator 
light, but both methods can have certain disadvantages. 
The use of an instrument is rather space-consuming 
particularly where a number of actuating devices are 
involved, and unless it is essential for a pilot or sys- 
tems engineer to know exactly the position of a device 
at any one time, instruments are uneconomical. Indi- 
cator lights are of course simpler, cheaper and consume 
less power, but the liability of their filaments to 
failure without warning contributes a hazard particu- 
larly in the case where "light out" is intended to 
indicate a "safe" condition of a system. Furthermore, 
in systems requiring a series of constant indications of 
prevailing conditions, constantly illuminated lamps 
can lead to confusion and misinterpretation on the 
part of the pilot or systems engineer. 

Therefore to enhance the reliability of indication, 
indicators containing small electromagnets operating 
a shutter or similar moving element are installed 
on the systems panels of many present-day aircraft. 

In its simplest form (see Fig. 6.1 1(a)) a magnetic 
indicator is of the two-position type comprising 
a ball pivoted on its axis and spring returned to the 
"off position. A ferrous armature embedded in the 
ball is attracted by the electromagnet when ener- 


gized, and rotates the ball through 150 degrees to 
present a different picture in the window. The picture 
can either be of the line diagram type, or of the 
instructive type. 


Plostic ball — 

- Return spring 

- Balance weight 

- Mognel assembly 


COftrO' SO'tnq Cabk qf0in met 

Mounting sQ'iiq 



Fig 6.1 1 

Magnetic indicators 

Figure 6. 1 1(b) shows a development of the basic 
indicator, it incorporates a second electromagnet 
which provides for three alternative indicating posi- 
tions. The ferrous armature is pivoted centrally above 
the two magnets and can be attracted by either of 
them. Under the influence of magnetic attraction the 
armature tilts and its actuating arm will slide the 
rack horizontally to rotate the pinions fixed to the 
ends of prisms. The prisms will then be rotated 
through 120 degrees to present a new pattern in the 
window. When the rack moves from the centre "rest" 
position, one arm of the hairpin type centring spring, 
located in a slot in the rack, will be loaded. Thus, if 
the electromagnet is de-energized, the spring will 
return to mid-position rotating the pinions and prisms 
back to the "off condition in the window. 

The pictorial presentations offered by these indi- 
cators is further improved by the painting of "flow 
lines" on the appropriate panels so that they inter- 
connect the indicators with the system control 
switches, essential indicators and warning lights. A 
typical application of magnetic indicators and "flow 
lines" is shown in Fig. 6.1. 

In the development of large types of aircraft and 
their associated systems, it became apparent that the 
use of warning and indicator lights in increasing num- 
bers, and widely dispersed throughout flight compart- 
ments, would present a problem and that a new 
approach would be necessary. As a result, systems 
referred to as "central warning systems" were 

In its basic form, a system comprises a group of 
warning and indicator lights connected to signal cir- 
cuits actuated by the appropriate systems of the air- 
craft, each light displaying a legend denoting the 
system, and a malfunction or advisory message. All 
the lights are contained on an annunciator panel 
installed within a pilot's visual range. In aircraft 
carrying a flight engineer, a panel is also installed at 
his station and is functionally integrated with the 
pilot's panel. A flight engineer's panel is illustrated in 
Fig. 6.12 and may be taken as an example of central 
warning displays. In this case, the panel is made up of 
a number of blue lights which are advisory of normal 
operating conditions, a number of amber lights, a red 
"master warning" light and an amber "master 
caution" light. 

When a fault occurs in a system, a fault-sensing 
device transmits a signal which illuminates the appro- 


priate amber light. The signal is also transmitted to an 
electronic device known as a logic controller, the 
function of which is to determine whether the fault 
is of a hazardous nature or is one requiring caution. 
If the fault is hazardous, then the controller output 
signal illuminates the red "master warning" light; 
if caution is required, then the signal will illuminate 
only the amber "master caution" light. 


8 ■ BLUE 
8 = RED 

Fig 6.12 
Centralized warning system annunciator panel 

Each master light incorporates a switch unit so 
that when the caps are pressed in, the active signal 
circuits are disconnected to extinguish the lights and, 
at the same time, they are reset to accept signals from 
faults which might subsequently occur in any other 
of the systems in the aircraft. The system lights are 
not of the resetting type and remain illuminated until 
the system fault is corrected. Dimming of lights and 
testing of bulb filaments is carried out by means of 
switches mounted adjacent to the annunciator panel. 

Test Questions 

1. Describe the operating principle of a moving coil 

2. Can moving coil instruments be directly connected 
in the circuits of a.c. systems for measurement of 
voltage and current, or is it necessary for them 

to be used with certain other components? 

3. A soft-iron core is placed within the coil of a moving 
coil instrument because: 

(a) it provides a solid spindle about which the coil 
can rotate. 

(b) this ensures an even, radial and intensified mag- 
netic field for the coil to move in. 

(c) the inertia of the core will damp out oscillations 
of the coil and pointer. 

4. Describe how ammeters can measure very high 
current values without actually carrying full load 

5. How are moving coil instruments protected against 
the effects of external magnetic fields? 

6. With the aid of a circuit diagram describe the 
operating principle of a frequency meter. 

7. With the aid of a block diagram, show how a 
centralized warning system may be used in a 
modern aircraft, list the systems which may be 
connected to the C.W.S. 



Power Distribution 

In order for the power available at the appropriate 
generating source, to be made available at the ter- 
minals of the power-consuming equipment then 
clearly, some organized form of distribution through- 
out an aircraft is essential. The precise manner in 
which this is arranged is governed principally by the 
type of aircraft and its electrical system, number of 
consumers and location of consumer components. For 
example, in a small light aircraft, electrical power 
requirements may be limited to a few consumer ser- 
vices and components situated within a small area, 
and the power may be distributed via only a few yards 
of cable, some terminal blocks, circuit breakers or 
fuses. In a large multijet transport aircraft on the 
other hand, literally miles of cable are involved, to- 
gether with multiple load distribution busbars, pro- 
tection networks, junction boxes and control panels. 


In most types of aircraft, the output from the genera- 
ting sources is coupled to one or more low impedance 
conductors referred to as busbars. These are usually 
situated in junction boxes or distribution panels 
located at central points within the aircraft, and they 
provide a convenient means for connecting positive 
supplies to the various consumer circuits; in other 
words, they perform a "carry-all" function. Busbars 
vary in form dependent on the methods to be adop- 
ted in meeting the electrical power requirements of 
a particular aircraft type. In a very simple system a 
busbar can take the form of a strip of interlinked 
terminals while in the more complex systems main 
busbars are thick metal (usually copper) strips or rods 
to which input and output supply connections can be 
made. The strips or rods are insulated from the main 
structure and are normally provided with some form 
of protective covering. Flat, flexible strips of braided 

copper wire are also used in some aircraft and serve 
as subsidiary busbars. 

Split Busbar Systems. The function of a distribution 
system is primarily a simple one, but it is complicated 
by having to meet additional requirements which con- 
cern a power source, or a power consumer system 
operating either separately or collectively, under 
abnormal conditions. The requirements and abnormal 
conditions, may be considered in relation to three 
main areas, which may be summarized as follows: 

1 . Power-consuming equipment must not be 
deprived of power in the event of power source 
failures unless the total power demand exceeds 
the available supply. 

2. Faults on the distribution system (e.g. fault 
currents, grounding or earthing at a busbar) 
should have the minimum effect on system 
functioning, and should constitute minimum 
possible fire risk. 

3. Power-consuming equipment faults must not 
endanger the supply of power to other equip- 

These requirements are met in a combined manner 
by paralleling generators where appropriate, by pro- 
viding adequate circuit protection devices, and by 
arranging for faulted generators to be isolated from 
the distribution system. The operating fundamentals 
of these methods are described elsewhere in this book, 
but the method with which this Chapter is concerned 
is the additional one of splitting busbars and distribu- 
tion circuits into sections in order to feed them from 
different sources. 

In adopting this method it is usual to categorize 
all consumer services into their order of importance 
and, in general, they fall into three groups: vital, 
essential and non-essential. 


Vital services are those which would be required 
after an emergency wheels-up landing, e.g. emergency 
lighting and crash switch operation of fire extin- 
guishers. These services are connected directly to the 

Essential services are those required to ensure safe 
flight in an in-flight emergency situation. They are 
connected to d.c. and a.c. busbars, as appropriate, 
and in such a way that they can always be supplied 
from a generator or from batteries. 

Non-essential services are those which can be 
isolated in an in-flight emergency for load shedding 
purposes, and are connected to d.c. and a.c. busbars, 
as appropriate, supplied from a generator. 

Figure 7.1 illustrates in much simplified form, the 

Fig 7.1 
Split busbar systems 

application of the foregoing split-busbar method to a 
power distribution system in which the power supplies 
are 28-volts d.c. from engine-driven generators, 115- 
volts 400 Hz a.c. from rotary inverters, and 28-volts 
d.c. from batteries. Each generator has its own busbar 
to which are connected the non-essential consumer 

services. Both busbars are in turn connected to a 
single busbar which supplies power to the essential 
services. Thus, with both generators operating, all 
consumers requiring d.c. power are supplied. The 
essential services busbar is also connected to the 
battery busbar thereby ensuring that the batteries 
are maintained in the charged condition. In the event 
that one generator should fail it is automatically 
isolated from its respective busbar and all busbar loads 
are then taken over by the operative generator. 
Should both generators fail however, non-essential 
consumers can no longer be supplied, but the batteries 
will automatically supply power to the essential ser- 
vices and keep them operating for a predetermined 
period calculated on the basis of consumer load re- 
quirements and battery state of charge. 

For the particular system represented by Fig. 7.1, 
the d.c. supplies for driving the inverters are taken 
from busbars appropriate to the importance of the 
a.c. operated consumers. Thus, essential a.c. con- 
sumers are operated by No. 1 inverter and so it is 
driven by d.c. from the essential services busbar. No. 
2 and No. 3 inverters supply a.c. to non-essential 
services and so they are powered by d.c. from the 
No. 1 and No. 2 busbars. 

Figure 7.2 illustrates another example of the split 
busbar method of power distribution, and is based on 
an aircraft utilizing constant-frequency a.c. as the 
primary power source and d.c. via transformer-rectifier 
units (T.R.U.'s). 

The generators supply three-phase power through 
separate channels, to the two main busbars and these, 
in turn, supply the non-essential consumer loads and 
T.R.U.'s. The essential a.c. loads are supplied from the 
essential busbar which under normal operating condi- 
tions is connected via a changeover relay to the No. 1 
main busbar. The main busbars are normally isolated 
from each other, but if the supply from either of the 
generators fails, the busbars are automatically inter- 
connected by the energizing of the "bus-tie" relay and 
serve as one, thereby maintaining supplies to all a.c. con- 
sumers and both T.R.U.'s. If, for any reason, the power 
supplied from both generators should fail the non- 
essential services will be isolated and the changeover 
relay between No. 1 main busbar, and the essential 
busbar, will automatically de-energize and connect 
the essential busbar to an emergency static inverter. 

The supply of d.c. is derived from independent 
T.R.U. and from batteries. The No. 1 T.R.U. supplies 
essential loads and the No. 2 unit supplies non- 
essential loads connected to the main d.c. busbar; 


No. I o.c.bus-bor 


( No 2 j 

qc consurne's 



Essential a c 


-o | o- 

Bus-tie relay 



Battery bus-bar 

— g — . J 

No I 

External External a.c 
power relay power 

oc. consumers 





Vital dc 


dc consumers 

dc consumer^ 

Fig 7.2 
Split busbar system (primary a.c power source) 

both busbars are automatically interconnected by 
an isolation relay. The batteries are directly connected 
to the battery busbar and this is interconnected with 
the essential busbar. In the event of both generators 
failing the main d.c. busbar will become isolated from 
the essential d.c. busbar which will then be auto- 
matically supplied from the batteries to maintain 
operation of essential d.c. and a.c. consumers. 

External power supplies and supplies from an 
auxiliary power unit (see also Chapter 5) can be con- 
nected to the whole system in the manner indicated 
in Fig. 7.2. 

Wires and Cables 

Wires and cables constitute the framework of power 
distribution systems conducting power in its various 
forms and controlled quantities, between sections 
contained within consumer equipment, and also 
between equipment located in the relevant areas of 
an aircraft. The differences between a wire and a 
cable relate principally to their constructional features 
(and indirectly to their applications also) and may be 
understood from the following broad definitions. 

A wire is a single solid rod or filament of drawn 
metal enclosed in a suitable insulating material and 
outer protective covering. Although the term properly 

refers to the metal conductor, it is generally under- 
stood to include the insulation and covering. Specific 
applications of single wires are to be found in con- 
sumer equipment; for example, between the supply 
connections and the brush gear of a motor, and also 
between the various components which together 
make up the stages of an electronic amplifier. 

A cable is usually made up of a conductor com- 
posed of a group of single solid wires stranded together 
to provide greater flexibility, and enclosed by insula- 
ting material and outer protective covering. A cable 
may be either of the single core type, i.e., with cores 
stranded together as a single conductor, or of the 
multicorc type having a number of single core cables 
in a common outer protective covering. 

In connection with power distribution systems in 
their various forms, such terms as "wiring systems", 
"wiring of components", "circuit wiring" are com- 
monly used. These are of a general nature and apply 
equally to systems incorporating either wires, cables 
or both. 


Wires and cables are designed and manufactured for 
duties under specific environmental conditions and 
are selected on this basis. This ensures functioning of 
distribution and consumer systems, and also helps to 


minimize risk of fire and structural damage in the 
event of failure of any kind. Table 7.1 gives details of 
some commonly used general service wires and cables 
of U.K. manufacture, while typical constructional 
features are illustrated in Fig. 7.3. 

The names adopted for the various types are 
derived from contractions of the names of the various 
insulating materials used. For example, "NYVIN" is 
derived from "NYlon" and from polyVINyl-chloride 
(P.V.C.); and "TERSIL" is derived from polyesTER 
and Silicone. Cables may also be further classified by 
prefixes and suffixes relating to the number of cores 
and any additional protective covering. For example, 
"TRINYVIN" would denote a cable made up of three 
single Nyvin cables, and if suffixed by "METSHEATH", 
the name would further denote that the cable is 
enclosed in a metal braided sheath. 

It will be noted from the Table that only two 
metals are used for conductors, i.e. copper (which 
may also be tinned, nickel-plated or silver-plated 
depending on cable application) and aluminium. 
Copper has a very low specific resistance and is 
adopted for all but cables of large cross-sectional 
areas. An aluminium conductor having the same 
resistance as a copper conductor, has only two-thirds 
of the weight but twice the cross-sectional area of 
the copper conductor. This has an advantage where 
low-resistance short-term circuits are concerned; for 
example, in power supply circuits of engine starter 
motor systems. 

The insulation materials used for wires and cables 
must conform to a number of rigid requirements such 
as, toughness and flexibility over a fairly wide tem- 
perature range, resistance to fuels, lubricants and 
hydraulic fluids, ease of stripping for terminating, non- 
fiammability and minimum weight. These require- 
ments, which are set out in standard specifications, are 
met by the materials listed in Table 7.1 and in the 
selection of the correct cable for a specific duty and 
environmental condition. 

To ensure proper identification of cables, standard 
specifications also require that cable manufacturers 
comply with a code and mark outer protective cover- 
ings accordingly. Such a coding scheme usually sig- 
nifies, in sequence, the type of cable, country of 
origin ("G" for U.K. manufacturers) manufacturer's 
code letter, year of manufacture also by a letter, and 
its wire gauge size, thus, NYVIN G-AN 22. A colour 
code scheme is also adopted particularly as a means of 
tracing the individual cores of multicore cables to and 
from their respective terminal points. In such cases it 
is usual for the insulation of each core to be produced 
in a different colour and in accordance with the 
appropriate specification. Another method of coding, 
and one used for cables in three-phase circuits of some 
types of aircraft, is the weaving of a coloured trace 
into the outer covering of each core; thus red — 
(phase A); yellow - (phase B); blue - (phase C). The 
code may also be applied to certain single-core cables 
by using a coloured outer covering. 



Nickel plated copper strand Silicone rubber 

Glass braid/PTP/varnish 


Tinned copper strand 

PVC Class braid 




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Nickel plated copper strand 





Fig 7.3 
Constructional features of some typical cables 




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As noted earlier in this chapter, the quantity of wires 
and cables required for a distribution system depends 
on the size and complexity of the systems. However, 
regardless of quantity, it is important that wires and 
cables be routed through an aircraft in a manner 
which, is safe, avoids interference with the reception 
and transmission of signals by such equipment as 
radio and compass sytems, and which also permits a 
systematic approach to their identification, installa- 
tion and removal, and to circuit testing. Various 
methods, dependent also on size and complexity, are 
adopted but in general, they may be grouped under 
three principal headings: (i) open loom, (ii) ducted 
loom, and (iii) conduit. 

Open Loom, In this method, wires or cables to be 
routed to and from consumer equipment in the 
specific zones of the aircraft, are grouped parallel to 
each other in a bundle and bound together with 
waxed cording or p.v.c. strapping. A loom is supported 
at intervals throughout its run usually by means of 
clips secured at relevant parts of the aircraft structure. 
An application of the method to an aircraft junction 
box is shown in Fig. 7.4. 

The composition of a cable loom is dictated by 
such factors as (i) overall diameter, (ii) temperature 
conditions, i.e. temperature rise in cables when opera- 
ting at their maximum current-carrying capacity in 
varying ambient temperature conditions, (iii) type of 
current, i.e. whether alternating, direct, heavy-duty or 
light-duty, (iv) interference resulting from inductive or 
magnetic effects, (v) type of circuit with which cables 
are associated; this applies particularly to circuits in 
the essential category, the cables of which must be 
safe-guarded against damage in the event of short- 
circuits developing in adjoining cables. 

Magnetic fields exist around cables carrying direct 
current and where these cables must interconnect 
equipment in the vicinity of a compass magnetic 
detector element, it is necessary for the fields to be 
cancelled out. This is achieved by routing the positive 
and earth-return cables together and connecting the 
earth-return cable at an earthing point located at a 
specific safe distance from the magnetic detector 
element of the compass. 

Ducted Loom This method is basically the same as 
that of the open loom except that the bundles are 
supported in ducts which are routed through the air- 
craft and secured to the aircraft structure (see Fig. 

Fig 7.4 
Open looms 

7.5). Ducts may be of aluminium alloy, resin- 
impregnated asbestos or moulded fibre-glass- 
reinforced plastic. In some applications of this 
method, a main duct containing several channels may 
be used, each channel supporting a cable loom cor- 
responding to a specific consumer system. For identi- 
fication purposes, each loom is bound with appropri- 
ately coloured waxed cording. 

Conduits are generally used for conveying cables in 
areas where there is the possibility of exposure to oil, 
hydraulic or other fluids. Depending on the particular 
application, conduits may take the form of either 
plastic, flexible metal or rigid metal sheaths. In cases 
where shielding against signal interference is necessary 
the appropriate cables are conveyed by metal conduits 
in contact with metal structural members to ensure 
good bonding. 

Cable Seals. In pressurized cabin aircraft it is essential 
for many cables to pass through pressure bulkheads 
without a "break" in them and without causing leakage 


Fig 7.5 
Ducted looms 

of cabin air. This is accomplished by sealing the neces- 
sary apertures with either pressure bungs or pressure- 
proof plugs and sockets. An example of a pressure 
bung assembly is shown in Fig. 7.6. It consists of a 
housing, perforated synthetic rubber bung, anti- 
friction washer and knurled clamping nuts; the hous- 

Clomp block 

Clomping Anti -friction 
nut washer 

Clomp support ' 


Fig 7.6 

Pressure bung assembly 

ing is flanged and threaded, having a tapered bore to 
accept the bung. The holes in the bung vary in size to 
accommodate cables of various diameters, each hole 
being sealed by a thin covering of synthetic rubber at 
the smaller diameter end of the bung. The covering is 
pierced by a special tool when loading the bung with 


The cables are a tight fit in the holes of the bung 
which, when fully loaded and forced into the housing 
by the clamping nut, is compressed tightly into the 
housing and around the cables. The anti-friction 
washer prevents damage to the face of the bung when 
the clamping nut is turned. On assembly, holes not 
occupied by cables are plunged with plastic plugs. 

In instances where cable "breaks" are required at a 
pressure bulkhead, the cables at each side of the bulk- 
head are terminated by specially-sealed plug or socket 
assemblies of a type similar to those shown in Fig. 
7. 1 1 (items 3 and 4). 

For certain types of electrical systems, cables are 
required to perform a more specialized function than 
that of the cables already referred to. Some examples 
of what are generally termed, special purpose cables, 
are described in the following paragraphs. 

Ignition Cables. These cables are used for the trans- 
mission of high tension voltages in both piston engine 
and turbine engine ignition systems, and are of the 
single-core stranded type suitably insulated, and 
screened by metal braided sheathing to prevent inter- 
ference. The number of cables required for a system 
corresponds to that of the sparking plugs or igniter 
plugs as appropriate, and they are generally made up 
into a complete ignition cable harness. Depending on 
the type of engine installation, the cables may be 
enclosed in a metal conduit, which also forms part of 
a harness, or they may be routed openly. Cables are 
connected to the relevant system components by 
special end fittings comprising either small springs or 
contact caps secured to the cable conductor, insulation, 
and a threaded coupling assembly. 

Thermocouple Cables. These cables are used for 
the connection of cylinder head temperature indicators 
and turbine engine exhaust gas temperature indicators 
to their respective thermocouple sensing elements. The 
conducting materials are normally the same as those 
selected for the sensing element combinations, namely, 
iron and constantan or copper and constantan for 


cylinder head thermocouples, chromel (an alloy of 
chromium and nickel) and alumel (an alloy of alumi- 
nium and nickel) for exhaust gas thermocouples. 

In the case of cylinder head temperature indicating 
systems only one thermocouple sensing element is 
used and the cables between it and a firewall connec- 
tor are normally asbestos covered. For exhaust gas 
temperature measurement a number of thermocouples 
are required to be radially disposed in the gas stream, 
and it is the usual practice therefore, to arrange the 
cables in the form of a harness tailored to suit a 
specific engine installation. The insulating material 
of the harness cables is either silicone rubber or 
P.T.F.E. impregnated fibre glass. The cables terminate 
at an engine or firewall junction box from which 
cables extend to the indicator. The insulating material 
of extension cables is normally of the polyvinyl type, 
since they are subject to lower ambient temperatures 
than the engine harness. In some applications exten- 
sion cables are encased in silicone paste within metal- 
braided flexible conduit. 

is then fanned out and folded back over the adapter 
(steps "B" and "C"). At the same time, the insulation 
is cut back to expose the inner conductor. The next 
step (D) is to screw the sub-assembly to the adapter 
thereby clamping the outer conductor firmly between 
the two components. Although not applicable to all 

Co-axial Cables. Co-axial cables contain two or more 
separate conductors. The innermost conductor may 
be of the solid, or stranded copper wire type, and may 
be plain, tinned, silver-plated or even gold-plated 
in some applications, depending on the degree of 
conductivity required. The remaining conductors are 
in the form of tubes, usually of fine wire braid. The 
insulation is usually of polyethylene or Teflon. Outer 
coverings or jackets serve to weatherproof the cables 
and protect them from fluids, mechanical and elec- 
trical damage. The materials used for the coverings 
are manufactured to suit operations under varying 
environmental conditions. 

Co-axial cables have several main advantages. First, 
they are shielded against electrostatic and magnetic 
fields; an electrostatic field does not extend beyond 
the outer conductor and the fields due to current 
flow in inner and outer conductors cancel each other. 
Secondly, since co-axial cables do not radiate, then 
likewise they will not pick up any energy, or be 
influenced by other strong fields. The installations in 
which coaxial cables are most commonly employed 
are radio, for the connection of antennae, and capaci- 
tance type fuel quantity indicating systems for the 
interconnection of tank units and amplifiers. The 
construction of a typical coaxial cable and also the 
sequence adopted for attaching the end fitting are 
shown in Fig. 7.7. The outer covering is cut back to 
expose the braided outer conductor (step "A") which 

Fig 7.7 
Typical coaxial cable and end fitting 

1. Outer braid conductor 

2. Outer covering 

3. Adapter 

4. Coupling ring 

5. Insulation 

6. Inner conductor 

7. Plug sub-assembly 

8. Contact 

9. Solder holes 

cables the outer conductor may also be soldered to the 

sub-assembly through solder holes. The assembly is 
completed by soldering a contact on to the inner con- 
ductor and screwing the coupling ring on to the sub- 


In the literal sense, earthing or grounding as it is often 
termed, refers to the return of current to the conduc- 
ting mass of the earth, or ground, itself. If considered 
as a single body, the earth is so large that any transfer 
of electrons between it and another body fails to pro- 
duce any perceptible change in its state of electrifi- 
cation. It can therefore be regarded as electrically 
neutral and as a zero reference point for judging the 
state of electrification of other bodies. For example, 
if two charged bodies, A and B, both have positive 
potentials relative to earth, but the potential of A is 
more positive than that of B, then the potential of B 
may be described as negative to that of A by the 
appropriate amount. 

As we have already learned, the positive outputs 
of aircraft power supplies and the positive input ter- 
minals of consumer components are all connected to 
busbars which are insulated from the aircraft struc- 
ture. Since in most aircraft the structure is of metal 
and of sufficient mass to remain electrically neutral, 
then it too can function as an earth or "negative 
busbar" and so provide the return path of current. 
Tlius, power supply and consumer circuits can be com- 
pleted by coupling all negative connections to the 
structure at various "earth stations", the number and 
locations of which are predicted in a manner appro- 
priate to the particular type of aircraft. As this results 
in the bulk of cable required for the circuits being on 
the positive side only, then such an electrical installa- 
tion is designated as a "single-wire, or single-pole, earth- 
return system". For a.c. power supply circuits the air- 
frame also serves as a connection for the neutral point. 

The selection of types of connection for earth 
return cables is based on such important factors as 
mechanical strength, current to be carried, corrosive 
effects, and ease with which connections can be made. 
As a result, they can vary in form; some typical 
arrangements being a single bolt passing through and 
secured directly to a structural member, and either a 
single bolt or a cluster or bolts secured to an earthing 
plate designed for riveting or bolting to a structural 
member. In order to ensure good electrical contact 
and minimum resistance between an earthing bolt or 
plate and the structure, protective film is removed from 
the contacting surfaces before assembly. Protection 
against corrosion is provided by coating (he surfaces 
with an anti-corrosion and solvent resistant compound 
or, in some cases by interposing an electro-tinned 
plate and applying compound to the edges of the 
joint. An example of a cluster arrangement with a 

corrosion plate is illustrated in Fig. 7.4. 

Earth-return cables are connected to earthing bolts 
by means of crimped ring type connectors, each bolt 
accommodating cables from several circuits. For some 
circuits, however, it is necessary to connect cables 
separately and this applies particularly to those of the 
sensitive low current-carrying type, e.g. resistance type 
temperature indicators in which errors can arise from 
varying earth return currents of other circuits. 

In aircraft in which the primary structure is of non- 
metallic construction, a separate continuous main 
earth and bonding system is provided. It consists of 
four or more soft copper strip-type conductors ex- 
tending the whole length of the fuselage and disposed 
so that they are not more than six feet apart as 
measured around the periphery of the fuselage at the 
position of greatest cross-sectional area. The fuselage 
earthing strips are connected to further strips which 
follow the leading and trailing edges from root to tip 
of each wing and horizontal stabilizer, and also to a 
strip located on or near the leading edge of the vertical 
stabilizer. Earthing strips are provided in the trailing 
edges of the rudder, elevators and ailerons, and are 
connected to the fuselage and wing systems via the 
outer hinges of the control surfaces. The strips are 
arranged to run with as few bends as possible and are 
connected to each other by means of screwed or 
riveted joints. 

Lightning strike plates, extending round the tips of 
each wing, horizontal and vertical stabilizers, fuselage 
nose and tail, are also provided. They consist of copper 
strips and are mounted on the exterior of the struc- 


In order to complete the linkages between the various 
units comprising a power distribution system, some 
appropriate means of connection and disconnection 
must be provided. The number of connections involved 
in any one system obviously depends on the type and 
size of an aircraft and its electrical installation, but 
the methods of connection with which we are here 
concerned follow the same basic pattern. 

In general, there are two connecting methods 
adopted and they can be broadly categorized by the 
frequency with wliich units must be connected or dis- 
connected. For example, cable connections at junc- 
tion boxes, terminal blocks, earth stations etc. are of 
a more permanent nature, but the cable terminations are 
such that the cables can be readily disconnected when 


occasion demands. With equipment of a complex 
nature liable to failure as the result of the failure of 
any one of a multitude of components, the connec- 
tions arc made by some form of plug and socket thus 
facilitating rapid replacement of the component. 
Furthermore, the plug and socket method also 
facilitates the removal of equipment that has to be 
inspected and tested at intervals specified in main- 
tenance schedules. 


There are several methods by which cable termina- 
tions may be made, but the one most commonly 
adopted in power distribution systems is the solder- 
less or crimped termination. The soldering method of 
making connections is also adopted but is more 
generally confined to the joining of internal circuit 
connections of the various items of consumer equip- 
ment and in some cases, to the connections between 
single-core cables and plug and socket contacts. 

Crimped Terminals. A crimped terminal is one 
which has been secured to its conductor by compres- 
sing it in such a way that the metals of both terminal 
and conductor merge together to form a homogeneous 
mass. Some of the advantages of the crimping method 

1. Fabrication is faster and easier, and uniform opera- 
tion is assured. 

2. Good electrical conductivity and a lower voltage 
drop is assured. 

3. Connections are stronger (approaching that obtained 
with cold welding); actually as strong as the conduc- 
tor itself. 

4. Shorting due to solder slop and messy flux problems 
are eliminated. 

5. "Wicking" of solder on conductor wires and "dry" 
joints are eliminated. 

6. When properly formed a seal against the ingress of 
air is provided and a corrosion-proof joint thereby 

A typical terminal (see Fig. 7.8) is comprised of 
two principal sections; crimping barrel and tongue. 
For a particular size of conductor the copper or alu- 
minium barrel is designed to fit closely over the barrel 
end of the conductor so that after pressure has been 
applied a large number of point contacts are made. 
The pressure is applied by means of a hand-operated 
or hydraulically-operated tool (depending on the size 
of conductor and terminal) fitted with a die, shaped to 
give a particular cross-sectional form, e.g. hexagonal. 

diamond or "W". The barrels are insulated by plastic 
sleeves which extend a short distance over the con- 
ductor insulation and provide a certain amount of 
support for the conductor allowing it to be bent in 
any direction without fraying of the conductor 
insulation or breaking of wire strands. In certain types 

Fig 7.8 

Crimped terminals 

1. Tongue 

2. Insulation sleeve 

3. Barrel 

4. Stainless steel support 
(laige-diameter cable terminals) 

of terminal the inside surface of the barrel is serrated 
so that under the crimping pressure the strands of the 
conductor "flow" into the serrations to make a con- 
nection of high tensile strength. The serrations have 
the additional function of assisting in the breaking 
down of the oxide layer that forms on conductor 
wires during the crimping operation. To facilitate 
inspection of the crimped joint, the barrel is fre- 
quently left open at the tongue end, or in some 
cases, is provided with an inspection hole through 
which sufficient insertion of the conductor into the 
barrel may be visually verified. 

The design of the tongue end depends on where 
and how the terminal is to be attached. The most 
common forms are the ring type and fork type. 

Where a connection between the ends of two cables 
has to be made, for example, in a cable run from the 
engine nacelle to the fuselage of an aircraft, a change 
from an efglas cable to a nyvin cable may be neces- 
sary, a variant of the crimped terminal is used. This 
variant is known as an in-line connector and consists 
essentially of two crimping barrels in series, one con- 
ductor entering and being crimped at each end. A 
plastic insulating sleeve is also fitted over the connec- 
tor and is crimped in position. 

A selection of terminals and in-line connectors are 
shown in Fig. 7.9. 

Aluminium Cable Connections. The use of alumin- 
ium wire as an electrical conductor for certain systems 
is due chiefly to the important weight advantage of 



Rg 7.9 
Terminals and in-line connectors 

this metal over copper. However, in order to acquire 
satisfactory electrical connections, certain installation 
techniques are necessary to compensate for two other 
principal characteristics of aluminium, namely the 
rapidity with which it oxidizes, and its softness. 

The oxide film is formed as soon as aluminium is 
exposed to the atmosphere and it not only acts as an 
insulator, but also increases in thickness as heat is 
generated by the flow of current, still further increas- 
ing the electrical resistance and causing corrosion at 
connecting joints. The method most commonly em- 
ployed for eliminating the oxide film is the one in 
which a special zinc granular compound is applied to 
the exposed ends of the cable and the appropriate ter- 
minal. Aluminium terminals are normally of the 
crimped type and the barrel is filled with compound; 
in some cases the barrel contains a pre-filled cartridge. 
When crimping takes place the compound is forced 
around and between the wire strands of the cable, and 
penetrates the oxide film to assist in breaking it down. 
In this manner, clean metal-to-metal contacts are pro- 
vided and the high electrical resistance of the oxide 
film is bypassed. Sealing of the terminal/cable joint 
is also achieved so that the oxide film cannot reform. 

In cases where an aluminium cable terminal is to 
be bolted directly to the aircraft structure, a busbar, or 
surface of a component, the surfaces are first cleaned 
and a coating of compound applied. To compensate 
for the relative softness of aluminium as compared 
with copper, fiat washers with larger diameters than 
the tongue end of a terminal arc used to help dis- 

tribute the clamping pressure over a wider surface. 
For reasons of softness also, tightening torques applied 
to bolted connections arc maintained within specific 

Plugs and Sockets. Plugs and sockets (or recep- 
tacles) are connecting devices which respectively con- 
tain male and female contact assemblies. They may 
be fixed or free items, i.e. fixed in a junction box, 
panel or a consumer component, or free as part of a 
cable to couple into a fixed item. There are many vari- 
ations in the design of plugs and sockets governed 
principally by the distribution circuit requirements, 
number of conductors to be terminated, and environ- 
mental conditions. In general, however, the conven- 
tional consi ruction follows the pattern indicated in 
exploded form in Fig. 7.10. The bodies or shells, are 
mostly of light alloy or stainless steel finished overall 
with a cadmium plating; they may be provided with 
either a male or female thread. Polarizing keys and 
keyways are also provided to ensure that plugs and 
sockets and their corresponding conductors, mate 
correctly; they also prevent relative movement 




soctoi insutotor 

Rg 7.10 
Plug and socket construction 


between their contacts and thereby strain, when the 
coupling rings are being tightened. The shells of "free''' 
plugs and sockets are extended as necessary by the 
attachment of outlets or endbells. These provide a 
means of supporting the cable or cable loom at the 
point of entry to the plug or socket thereby pre- 
venting straining of the conductor, and pin or socket 
joints, they prevent displacement of the contacts in 
the softer material insulators, and the ingress of 
moisture and dirt. In many cases a special cable clamp 
is also provided (see Fig. 7.1 1, item 5). 

Plug contacts are usually solid round pins, and 


Fig 7.11 

Typical plugs and sockets 

(1) Fixed equipment and panel type 

(2) Fixed through-type bulkhead 

(3) Free type with cable clamp 

(4) Fixed type angle fitting 

(5) Fixed type rack equipment 

(6) Fixed type angle fitting 

(7) Fixed type rack equipment 

socket contacts have a resilient section which is 
arranged to grip the mating pin. The contacts are re- 
tained in position by insulators or inserts as they are 
often called, which are a sliding fit in the shells and 
are secured by retaining rings and/or nuts. Insulators 
may be made from hard plastic, neoprene of varying 
degrees of hardness, silicone rubber or fluorosilicone 
rubber depending on the application of a plug and 
socket, and on the environmental conditions under 
which they are to be used. Attachment of conductors 
to pin and socket contacts is done by crimping (see 
p. 90) a method which has now largely superseded that 
of soldering. The socket contacts are designed so that 
their grip on plug pin contacts is not reduced by 
repeated connection and disconnection. 

In most applications, plugs and sockets are secured 
in the mated condition by means of threaded 
coupling rings or nuts; in some cases bayonet-lock 
and push-pull type couplings may also be employed. 

Some typical fixed and free type plugs and 
sockets are illustrated in Fig. 7.11. The rack type 
units (items 6 and 7) are used principally for the inter- 
connection of radio and other electronic equipment 
which are normally mounted in special racks or trays. 
One of the units, either the plug or the socket, is fixed 
to the back of the equipment and the mating unit 
is fixed to the rack or tray; electrical connection is 
made when the equipment is slid into the rack or 


This is a technique usually applied to plugs and soc- 
kets which are to be employed in situations where 
there is the possibility of water or other liquids 
passing through the cable entry. It eliminates elaborate 
cable ferrules, gland nuts, etc, by providing a simple 
plastic shroud with sufficient height to cover the 
terminations, and filling the cavity with a special 
compound which though semi-fluid in its initial con- 
dition, rapidly hardens into a rubbery state to form a 
fairly efficient seal. In addition to sealing it provides 
reinforcement for the cable connections. 

The potting compound consists of a basic material 
and an alkaline or acid base material (known as an 
"accelerator") which are thoroughly mixed in the 
correct proportion to give the desired consistency and 
hardness of the compound. Once mixed, the compound 
is injected into a special mould and allowed to set. 
When the mould is removed, the resilient hemispheri- 
cally-shaped insulation extends well into the plug or 
socket, bonding itself to the back of the insulant 


around the contact and conductor joints and partly 
out along the conductor insulation. 

Electrical Bonding 


During flight, a build-up of electrical energy occurs in 
the structure of an aircraft, developing in two ways: 
by precipitation static charges and by charges due to 
electrostatic induction. Precipitation static charges are 
built up on the outer surfaces of an aircraft due to 
frictional contact with rain particles, snow and ice 
crystals, dust, smoke and other air contamination. 
As the particles flow over the aircraft negative charges 
are left behind on the surfaces and positive charges 
are released to flow into the airstream. In addition, 
particles of foreign impurities which are themselves 
charged, make physical contact and transfer these 
charges to the surfaces of the aircraft, increasing or 
decreasing the charged state already present by virtue 
of the frictional build-up. 

Charges of the electrostatic type are those induced 
into an aircraft when flying into electric fields created 
by certain types of cloud formation. This condition 
of charge is the result of the disruption of water par- 
ticles which increases the strength of a field and builds 
up such a high voltage that a discharge occurs in the 
familiar form of lightning. The discharge can take 
place between oppositely charged pockets in one 
cloud or a negatively charged section and the top of 
the cloud, or between a positively charged pocket 
and earth or ground. A well developed cloud may have 
several oppositely charged areas, which will produce 
several electric fields in both the horizontal and ver- 
tical planes, where voltages of up to 10,000 volts per 
centimetre can be achieved. The relative hazard created 
by these high potentials can be readily appreciated 
if it is realized that by electrostatic induction, up to 
10 million volts with possibly several thousand am- 
peres of current, may be permitted to pass through 
the aircraft when flying in or near the aforementioned 

Regardless of how an aircraft acquires its static 
charges the resultant potential difference between it 
and the atmosphere produces a discharge which tends 
to adjust the potential of the aircraft to that of the 
atmosphere. The charge is therefore being dissipated 
almost as it is being acquired, and by natural means. 

One of the hazards, however, is the possibility of 
discharges occurring within the aircraft as a result of 
differences between the potentials of the separate 

parts which go to make up the aircraft, and all the 
systems necessary for its operation. It is essential, there- 
fore, to incorporate a system which will form a 
continuous low-resistance link between all parts and 
in so doing will: 

(i) limit the potential difference between all 

(ii) eliminate spark discharges and fire risks, 
(iii) carry the exceptionally high voltages and 
currents so that they will discharge to atmo- 
sphere at the extremities of the aircraft, 
(iv) reduce interference with radio and navi- 
gational aid signals. 
(v) prevent the possibility of electrical shock 
hazards to persons contacting equipment 
and parts of the aircraft. 
Such a system is called a bonding system and 
although differing in its principal functions, it will be 
clear from the fact that electrical continuity is ob- 
tained, the requirements of the system overlap those 
of the earthing system described on p. 89. 

The continuous link is formed by metal strip 
conductors joining fixed metal parts, e.g. pipes joined 
either side of a non-metallic coupling, and by short- 
length flexible braid conductors for joining moving 
parts such as control rods, flight control surfaces, and 
components mounted on flexible mountings, e.g. 
instrument panels, mounting racks for electronic 
equipment. Some typical examples of the method of 
joining bonding strips or "jumpers" as they are some- 
times called, are shown in Fig. 7.12. 

In general, bonding is classified as Primary and 
Secondary, such classifications being determined by 
the magnitude of current to be expected from electro- 
statically induced charges, and precipitation static 
charges respectively. Primary bonding conductors 
are used between major components, engines, external 
surfaces, e.g. flight control surfaces, and the main 
structure or earth. Secondary bonding conductors are 
used between components and earth for which pri- 
mary conductors are not specifically required, e.g. 
pipelines carrying flammable fluids, metal conduits, 
junction boxes, door plates, etc. 

Some static charge is always liable to remain on an 
aircraft so that after landing a difference in potential 
between the aircraft and the ground could be caused. 
This obviously is undesirable, since it creates an elec- 
tric shock hazard to persons entering or leaving the 
aircraft, and can cause spark discharge between the 
aircraft and external ground equipment being coupled 
to it. In order to provide the necessary leakage path, 

Metal clip 

Bonding leod of sufficient 
length to permit lever 

Metal clip 

(a) Levers and control rods 

Bonding strip 

'Pipe clip 

Rubber hose clip 

(b) Pipes with non-metallic couplings 

Main surface 

Flight control 

*■"*' surface 

c) Flight control sufaces 



(d) Flexible coupling at bulkheods 

Bonding lead 
spiralled priori 
to fitting 

Shock absorber 

(e) ShoCK-mounfed equipment 

Fig 7.12 
Bonding methods 


two methods are generally adopted either separately 
or in combination. In one, the aircraft is fitted with a 
nosewheel or tail-wheel tyre as appropriate, the 
rubber of which contains a compound providing the 
tyre with good electrical conductivity. The second 
method provides a leakage path via short flexible steel 
wires secured to the nose wheel or main wheel axle 
members and making physical contact with the ground. 

During refuelling of an aircraft, stringent pre- 
cautions are necessary to minimize the risk of fire or 
explosion due to the presence of static charges. The 
aircraft itself may be charged, the fuel flowing 
through the hose generates electrical potentials, and 
the fuel tanker may be charged. Thus potential dif- 
ferences must be prevented from occurring and which 
could otherwise result in the generation of sparks and 
ignition of flammable vapours. The equalizing of 
potentials is achieved by providing a bonding connec- 
tion between the aircraft and tanker which themselves 
are bonded to the ground, and by bonding the hose 
nozzle to a point specially provided on the aircraft. 
During the refuelling operation physical contact 
between the hose nozzle and tank filler is always 

As noted earlier, the discharge of static takes place 
continuously in order to equalize the potentials of the 
charges in the atmosphere and the aircraft. However, 
it is often the case that the rate of discharge is lower 
than the actual charging rate, with the result that the 
aircraft's charge potential reaches such a value it per- 
mits what is termed a corona discharge, a discharge 
which if of sufficient magnitude, will glow in poor 
visibility or at night. Corona discharge occurs more 
readily at curves and sections of an aircraft having 
minimum radii such as wing tips, trailing edges, 
propeller tips, horizontal and vertical stabilizers, 
radio antennae, pitot tubes, etc. 

Corona discharge can cause serious interference 
with radio frequency signals and means must therefore 
be provided to ensure that the discharges occur at 
points where interference will be minimized. This is 
accomplished by devices called static discharge wicks 
or more simply, static dischargers. They provide a 
relatively easy exit for the charge so that the corona 
breaks out at predetermined points rather than hap- 
hazardly at points favourable to its occurrence. Static 
dischargers are fitted to the trailing edges of ailerons, 
elevators and rudder of an aircraft. A typical static 
discharger consists of nichrome wires formed in the 

manner of a brush or wick thereby providing a number 
of discharge points. In some instances, static dis- 
chargers may also take the form of small metal rods 
for trailing edge fitting and short fiat metal blades for 
fitting at the tips of wings, horizontal and vertical 
stabilizers. Sharp tungsten needles extend at right 
angles to the discharger tips to keep corona voltage 
low and to ensure that discharge will occur only at 
these points. 


Screening performs a similar function to bonding in 
that it provides a low resistance path for voltages 
producing unwanted radio frequency interference. 
However, whereas a bonding system is a conducting 
link for voltages produced by the build up of static 
charges, the voltages to be conducted by a screening 
system are those stray ones due to the coupling of 
external fields originating from certain items of 
electrical equipment, and circuits when in operation. 
Typical examples are: d.c. generators, engine ignition 
systems, d.c. motors, time switches and similar 
apparatus designed for making and breaking circuits 
at a controlled rate. 

The methods adopted for screening are generally 
of three main types governed principally by the equip- 
ment or circuit radiating the interference fields. In 
equipment such as generators, motors and time 
switches several capacitors, which provide a low resis- 
tance path, are interconnected across the inter- 
ference source, i.e. brushes, commutators and con- 
tacts, to form a self-contained unit known as a sup- 
pressor. The other methods adopted are the enclosing 
of equipment and circuits in metal cases and the en- 
closure of cables in a metal braided sheath, a method 
used for screening the cables of ignition systems. The 
suppressors and metal screens are connected to the 
main earth or ground system of an aircraft. 

Electrical Diagrams and Identification Schemes 

As in all cases involving an assembly, interconnection 
and maintenance, of a number of components forming 
a specific system, a diagram is required to provide the 
practical guide to the system. Aircraft electrical instal- 
lations are, of course, no exception to this requirement 
and the relevant drawing practices arc specialized sub- 
jects necessitating separate standardization of detail to 
ensure uniformity in preparation and presentation. 
The standards to which all diagrams are normally 
drawn are those laid down by appropriate national 


organizations, e.g. Ihe British Standards Institution, 
Society of British Aerospace Constmctors (S.B.A.C.) 
and in Specification 100 of the Air Transport Associ- 
ation (A.T.A.) of America. There arc usually three 
types of diagram produced for aircraft namely, circuit 
diagrams, wiring diagrams and routing charts. 

nature in that they show how all components and 
cables of each individual system making up the whole 
installation, are to be connected to each other, their 
locations within the aircraft and groups of figures and 
letters to indicate how all components can be identi- 
fied directly on the aircraft (see also p. 97). 

Circuit Diagrams. These are of a theoretical nature 
and show the internal circuit arrangements of electrical 
and electronic components both individually and 
collectively, as a complete distribution or power con- 
sumer system, in the detail necessary to understand 
the operating principle of the components and system. 
Circuits are normally drawn in the "aircraft-on-the- 
ground" condition with the main power supply off. In 
general, switches are drawn in the "off position, and 
all components such as relays and contactors are 
shown in their demagnetized state. Circuit breakers 
are drawn in the closed condition. In the event that 
it is necessary to deviate from these standard conditions, 
a note is added to the diagram to clearly define the con- 
ditions selected. 

Wiring Diagrams. These are of a more practical 

Routing Charts. These charts have a similar function 
to wiring diagrams, but are set out in such a manner 
that components and cables are drawn under 
"location" headings so that the route of distribution 
can be readily traced out on the aircraft. In some 
cases, both functions may be combined in one diagram 
(see Fig. 7.13). 

Wiring diagrams and routing charts are provided for 
the use of maintenance engineers to assist them in 
their practical tasks of testing circuits, fault finding 
and installation procedures. The number of diagrams 
or charts required for a particular aircraft, obviously 
depends on the size of the aircraft and its electrical 
installation, and can vary from a few pages at the end 
of a maintenance manual for a small light aircraft, to 
several massive volumes for large transport aircraft. 



Panel Sr. 

Zone F2 

S-jC-Donel E 




M.°2 intake overneat 
warning lamp 



N°2 debus 

|"kn\K)-^— 2WGI A22- 

• 2WGIB22 - 

Bung"! Bung 
GC2 , GC3 



Zore ~ 
Frame 55 

Jndera • intake ■ 

sensitive switch 

Fig 7.13 
Hiactical application of coding 



As an aid to the correlation of the details illustrated 
in any particular diagram with the actual physical 
conditions, i.e. where items are located, sizes of 
cables used, etc., aircraft manufacturers also adopt an 
identification coding scheme apart from those adopted 
by cable manufacturers. Such a scheme may either be 
to the manufacturer's own specification, or to one 
devised as a standard coding scheme. In order to 
illustrate the principles of schemes generally, some 
example applications of one of the more widely adop- 
ted coding standards will be described. 

In this scheme, devised by the Air Transport Associ- 
ation of America under Specification No. 100, the 
coding for cable installations consists of a six-position 
combination of letters and numbers which is quoted 
on all relevant wiring diagrams and routing charts and 
is imprinted on the outer covering of cables. In cases 
where the code cannot be affixed to a cable it is 
printed on non-metallic sleeves placed over the ends 
of the cable. The code is printed at specified intervals 
along the length of a cable by feeding it through a 
special printing machine. The following example serves 
to illustrate the significance of each position of the 

1 P 1 A 22 N 

Position 1. The number in this position is called the 
unit number and is only used where components have 
identical circuits, e.g., the components of a twin 
generator system. In this case number 1 refers to the 
cables interconnecting the components of the first 
system. The number is omitted from cables used 

Position 2. In this position, a letter is used to indicate 
the function of the circuit i.e., it designates the circuit 
or system with which the cable is connected. Each 
system has its own letter. When the circuit is part of 
radar, radio, special electronic equipment, a second 
letter is used to further define the circuit. 

Position 3. The number in this position is that of the 
cable and is used to differentiate between cables which 
do not have a common terminal in the same circuit. 
In this respect, contacts of switches, relays, etc. are 
not classified as common terminals. Beginning with 
the lowest number and progressing in numerical order 
as far as is practicable, a different number is given to 
each cable. 

Position 4. The letter used in this position, signifies the 
segment of cable (i.e., that portion of cable between 
two terminals or connections) and differentiates 
between segments in a particular circuit when the 
same cable number is used throughout. When practic- 
able, segments are lettered in alphabetical sequence 
(excluding the letter "I" and "O") the letter "A" 
identifying the first segment of each cable, beginning 
at the power source. A different letter is used for 
each of the cable segments having a common terminal 
or connection. 

Position 5. In this position, the number used indicates 
the cable size and corresponds to the American Wire 
Gauge (AWG) range of sizes. This does not apply to 
coaxial cables for which the number is omitted, or to 
thermocouple cables for which a dash ( - ) is used as 
a substitute. 

Position 6. In this position, a letter indicates whether 
a cable is used as a connection to a neutral or earth 
point, an a.c. phase cable, or as a thermocouple cable. 
The letter "N" indicates an earth-connected cable, the 
letter "V" indicates a supply cable in a single-phase 
circuit, while in three-phase circuits the cables are 
identified by the letters "A", "B" and "C". Thermo- 
couple cables are identified by letters which indicate 
the type of conductor material, thus: AL (Alumel); 
CH (Chromel); CU (Copper); CN (Constantan). 

The practical application of the coding scheme may 
be understood from Fig. 7.13 which shows the wiring 
of a very simple temperature sensing switch and warn- 
ing lamp system. The diagram also serves as an 
example of how interconnections may be presented 
on the routing chart principle, the locations of com- 
ponents and cables being indicated along the top of 
the diagram. 

The system is related to the No. 2 engine air intake, 
its circuit function is designated by the letters "WG", 
and it uses cables of wire size 22 throughout. Starting 
from the power source i.e., from the No. 2 d.c. busbar, 
the first cable is run from the fuse connection 2, 
through a pressure bung to terminal 1 of the switch; 
thus, the code for this cable is 2 WG 1 A 22. Terminal 
1 also serves as a common power supply connection to 
the contact 2 of the prcss-to-test facility in the warn- 
ing lamp; therefore, the interconnecting cable which 
also passes through a pressure bung, is a second seg- 
ment cable and so its code becomes 2 WG 1 B 22. 
Terminal 2 of the switch serves as a common con- 


nection for the d.c. output from both contact 1 of 
the press-to-test facility, and the sensing switch con- 
tacts, and as the cables are the second pair in the cir- 
cuit and respectively first and second segments, their 
code numbers are 2 WG 2 A 22 and 2 WG 2 B 22. The 
cable shown going away from the B+ terminal of the 
lamp, is a third segment connecting a supply to a 
lamp in a centralized warning system and so accord- 
ingly carries the code 2 WG 2 C 22. The circuit is com- 
pleted via cable number 3 and since it connects to 
earth it carries the full six-position code; thus, 2 WG 
2 A 22 N. 

The coding schemes adopted for items of electrical 
equipment, control panels, connector groups, junction 
boxes, etc, are related to physical locations within the 
aircraft and for this purpose aircraft are divided into 
electrical zones. A reference letter and number are 
allocated to each zone and also to equipment, 
connectors, panels etc., so that they can be identified 
within the zones. The reference letters and numbers 
are given in the appropriate wiring diagrams and are 
correlated to the diagrammatic representations of all 
items (see Fig. 7.13). In the aircraft itself, references 
are marked on or near the related items. 

Test Questions 

1. What is the function of busbars and what form do 
they normally take? 

2. What is meant by a split busbar system? 

3. Define the three groups which usually categorize 
the importance of consumer services. 

4. Describe three different types of electrical cable 

in common aircraft use, stating their properties, 
limitations and identifications. State a typical use 
for each. (S.L.A.E.T.) 

5. What are the three principal methods of routing 
cables through an aircraft? 

6. Describe a method of routing wires and cables 
from a pressurized to a non-pressurized area of an 

7. Name some of the materials used for thermocouple 
cables and state their applications. 

8. What is meant by earthing or grounding? 

9. How is an earth system formed in an aircraft the 
primary structure of which is non-metallic? 

10. What is a crimped terminal? 

11. What is the function of an in-line connector? 

12. What precautions must be taken when making 
aluminium cable connections? 

1 3. How is it ensured that a plug mates correctly with 
its socket? 

14. Discuss briefly the process of "potting" a cable 
to a plug or socket. 

15. What are the principal functions of a bonding 

16. State some of the applications of primary and 
secondary bonding. 

1 7. Briefly describe the methods generally adopted 
for the discharging of static. 

1 8. What is the purpose of screening? 

1 9. Which figure and letter groups of the cable code 
3-R-123-S-20-C indicate (i) the cable size, (ii) the 
circuit function and (iii) a segment of cable? 

20. What letter is used in a code to indicate that the 
cable is part of a single-phase a.c. supply circuit? 


Circuit Controlling Devices 

In aircraft electrical installations the function of 
initiating, and subsequently controlling the operating 
sequences of constituent circuits is performed prin- 
cipally by switches and relays, and the construction 
and operation of some typical devices form the subject 
of this chapter. It may be noted that although circuit 
breakers may also come within the above functional 
classification, they are essentially circuit protection 
devices and, as such, are separately described in the 
appropriate chapter. 


Switches and relays are constructed in a variety of 
forms, and although not exhaustive, the details given in 
Table 8. 1 may be considered a fairly representative 
summary of the types and the actuating methods com- 
monly employed. 


In its simplest form, a switch consists of two contact- 
ing surfaces which can be isolated from each other 


Switching Device 

Primary method of actuating contact assemblies 












Certain types 
incorporate a 
"hold-in" coil; 











Mechanical timing 
device operated in 
turn by an electric 








Effects of metal 
expansion and also 
of electric current. 






Electromagnetic, in 
turn controlled by 



a circuit incorpora- 
ting one or more 
manual switches, 
switches or a com- 
bination of these. 


or brought together as required by a movable con- 
necting link. This connecting link is referred to as a 
pole and when it provides a single path for a flow of 
current as shown in Fig. 8.1(a), the switch is desig- 
nated as a single-pole, single-tlvow switch. The term 
throw thus indicates the number of circuits each pole 
can complete through the switch. In many circuits, 
various switching combinations are usually required, 
and in order to facilitate the make and break opera- 
tions, the contact assemblies of switches (and certain 
relays) may be constructed as integrated units. For 
example, the switch at (b) of Fig. 8.1 can control 
two circuits in one single make or break operation, and 
is therefore known as a double-pole, single-throw 
switch, the poles being suitably insulated from each 
other. Two further examples are illustrated in diagrams 
(c) and (d) and are designated single-pole, double- 
throw and double-pole, double-throw respectively. 

purpose" switching functions and are used extensively 
in the various circuits. A typical switch is illustrated in 
Fig. 8.2. 

toggle seal 

i o) Single -pole, single-throw 

(D) Double-pole, smgle-lhrow 

Fig 8.2 
Toggle switch 

In some applications it may be necessary for the 
switches in several independent circuits to be actuated 
simultaneously. This is accomplished by "ganging" the 
switches together by means of a bar linking each 
toggle as shown in Fig. 8.3(a). A variation of this 
method is used in certain types of aircraft for simul- 
taneous action of switch toggles in one direction only 

t C J Singie -pole , dojble-f Ivow 

i a) Double-poie, doabie-liwow 

Fig 8.1 

Switch contact arrangements 

In addition to the number of poles and throws, 
switches (toggle types in particular) are also desig- 
nated by the number of positions they have. Thus, a 
toggle switch which is spring-loaded to one position 
and must be held at the second to complete a circuit, 
is called a single-position switch. If the switch can be 
set at either of two positions, e.g. opening the circuit 
in one position and completing it in another, it is then 
called a two-position switch. A switch which can be 
set at any one of three positions, e.g. a centre "off 
and two "on" positions, is a tliree-position switch, 
also known as a selector switch. 


Toggle or tumbler-type switches, as they are sometimes 

called, perform what may be regarded as "general- 

^ JE 


Fig 8.3 

"Ganging" and locking of switches 


(usually to a "system off position). This is accom- 
plished by a separate gang-bar mounted on the control 
panel in such a way that it can be pulled down to bear 
against the toggles of the switches to push them in the 
required direction. When the bar is released it is re- 
turned under the action of a spring. 

A further variation is one in which the operation of 
a particular switch, or all in a series, may be constrained. 
A typical application to a triple generator system is 
shown in Fig. 8.3(b), the switches being used for the 
alternative disposition of busbar loads in the event of 
failure of any of the three generators. 

A locking bar is free to rotate in mounting brackets 
anchored by the locking nuts of the No. 1 and No. 2 
switches. The radiused cut-outs, at 90 degrees to each 
other, are provided along the length of the bar at 
positions coincident with the toggles of each switch. A 
steel spring provides for tensioning of the bar at each 
selected position, and is inserted around the circum- 
ference at the right-hand end. Markings 1 , 2, 3 and 
"N" correspond to the positions of the cut-outs on the 
bar 'dative to the switch toggles. If, for example, there 
is a failure of No. 1 generator the bar is rotated to the 
position 1 permitting operation of failure switch No. 1, 
but constraining the toggles of the other two switches. 
The action for switch operation at positions 2 and 3 
is similar. Thus, the busbar loads of a failed generator 
can be distributed between remaining serviceable 
generators at the same time avoiding inadvertent switch 
operation. When the letter "N" is evident the bar and 
the cut-outs are positioned so that none of the switches 
can be operated. 


Push-switches arc used primarily for operations of 
short duration, i.e. when a circuit is to be completed 
or interrupted momentarily, or when an alternative 
path is to be made available for brief periods. Other 
variants are designed to close one or more circuits 
(through separate contacts) while opening another 
circuit, and in these types, provision may be made for 
contact-action in the individual circuits to occur in 
sequence instead of simultaneously. In basic form a 
push-switch consists of a button-operated spring- 
loaded plunger carrying one or more contact plates 
which serve to establish electrical connection between 
fixed contact surfaces. Switches may be designed as 
independent units for either "push-to-makc" or "push- 
to-break" operation, or designed to be double-acting. 
For certain warning and indicating purposes, some 
types contain miniature lamps positioned behind a 

small translucent screen in the push-button. When 
illuminated, legends such as "on", "closed" or "fail" 
are displayed on the screen and in the appropriate 

The construction of a simple type of "push-to- 
make" switch and the arrangement of an illuminated 
type are shown in Fig. 8.4. In some circuits, for 
example in a turbopropeller engine starting circuit 
(see also p. 140), switches are designed to be both 
manual and electromagnetic in operation. A typical 
example, normally referred to as a "push-in solenoid 

Terminal screw 

Clomping ring 

Pusr. Ouilon 

Terminal cover 

Conroc - note 
Simple type 

Switch Mousing 

Switch octuating plunger 

Iilummoleo lens 

Lamp contacts 

Illuminoted type 

Fig 8.4 
Push switches 

switch", is shown in Fig. 8.5. The components are 
contained within a casing comprising an aluminium 
housing having an integral mounting flange, a sleeve 
and an end cover. The solenoid coil is located at the 
flange-end of the housing, and has a plunger passing 
through it. One end of the plunger extends beyond 
the housing flange and has a knob secured to it, while 
the other end terminates in a spring-loaded contact 
assembly. A combined terminal and fixed contact 
block is attached to the end of the housing and is held 
in place by a knurled end cover nut. 



Contact ossembly 

End cover nut 

-End cover 

Terminal and fixed 
contact block 

Fig 83 
Push-in solenoid switch 

When the plunger is depressed and held, the spring- 
loaded contact assembly bears against the fixed con- 
tacts and connects a d.c. supply to the starter motor. 
The commencement of the starting cycle provides a 
current flow through the hold-in coil of the switch, 
thereby energizing it and obviating the necessity for 
further manual control. The switch remains in the 
"on" position until the starting cycle is completed. 
At this stage, the current through the solenoid coil 
will have dropped sufficiently to permit the spring to 
return the plunger and contacts tc the "off" position. 

Rocker-button switches combine the action of both 
toggle and push-button type switches and are utilized 
for circuit control of some systems and equipment. 
A typical switch is shown in section in Fig. 8.6. For 
certain warning and indicating purposes, some types 
are provided with a coloured cap or screen displaying 
legend information, illuminated by a miniature lamp. 

operation. Furthermore, the rotary principle and 
positive engagement of contacts made possible by the 
constructional features make these switches more 
adaptable to multi-circuit selection than toggle type 
switches. A typical application is the selection of a 
single voltmeter to read the voltages at several bus- 
bars. In the basic form a rotary switch consists of a 

Removoble plostic 


One-piece C09t 
busnmg ond 

switching chombei 

Molded in 
Terminal ;nsert 

eiostome- seol 



Cover seol 
Locking nuT 

High slrength 
Temperature resistanr 
plastic case 

Fig 8.6 
Rocker-button switch 


These are manually operated, and for certain operating 
requirements they offer an advantage over toggle 
switches in that they are less prone to accidental 

central spindle carrying one or more contact plates or 
blades which engage with corresponding fixed con- 
tacts mounted on the switch base. The movement is 
usually spring-loaded and equipped with some form 



of eccentric device to give a snap action and positive 
engagement of the contact surfaces. 


Micro-switches are a special category of switch and 
are one of the most extensively applied electrical 
devices in aircraft, performing a wide range of opera- 
tions to ensure safe control of a variety of systems 
and components. The term "micro-switch" designates 
a switching device in which the differential travel 
between "make" and "break" of the operating mech- 
anism is of the order of a few thousandths of an inch. 
Magnification and snap action of contact mechanism 
movements are derived from a pre-tensioned mechan- 
ically biased spring. The principle is shown in Fig. 8.7. 

Spring (long member) 
Operating plunger / F »ced contact t normally closed) 

Fixed contoct 
[normol'y ooen) 

Common termina 

Side members 
Seoled housing 

Rol ••■■ 

^Ro'ler guide 

OC< ring 

Plunge' actuator 


Locking wire hole 

Bushing keywoy 

Steel coo 


Fig 8.7 


The long member of the one-piece spring is cantilever 
supported and the operating button or plunger bears 
against the spring. Two shorter side members are 
anchored in such a way thai they are bowed in com- 
pression. In the inoperative position the contact 
mounted on the free end of the spring is held against 
the upper fixed contact by the couple resulting from 
both tension and compression force. Depression of the 

operating button deflects the long member downwards 
thereby causing a reversal of the couple which "snaps" 
the spring and contact downward. Upon removal of 
the operating force, cantilever action restores the 
spring and contact system to its initial position with a 
snap action. 

The method of actuating micro-switches depends 
largely on the system to which it is applied but 
usually it is either by means of a lever, roller or cam, 
these in turn being operated either manually or elec- 
trically. The operating cycle of a micro-switch is 
defined in terms of movement of the operating 
plunger. This has a specified amount of pre-travel, or 
free movement before the switch snaps over. Follow- 
ing the operating point, there is some over-travel, 
while on the return stroke some differential travel 
beyond the operating point is provided before the 
release action of the switch takes place. The contacts 
of the switches are shown in Fig. 8.7, and these 
operate within sealed evacuated chambers filled with 
an inert gas, e.g. nitrogen. 


These are controlling devices containing a resistance 
the magnitude of which can be varied, thereby adjus- 
ting the current in the circuit in which it is connected. 
A typical example of this method of control is the 
one adopted for varying the intensity of instrument 
panel and certain cockpit lighting. 

Rheostats normally adjust circuit resistance with- 
out opening the circuit, although in some cases, they 
are constructed to serve as a combined on-off switch 
and variable resistor. 


Certain consumer services are required to operate on a 
pre-determined controlled time sequence basis and as 
this involves the switching on and off of various com- 
ponents or sections of circuit, switches automatically 
operated by timing mechanisms are necessary. The prin- 
ciple of time switch operation varies, but in general 
it is based on the one in which a contact assembly is 
actuated by a cam driven at constant speed by either 
a speed-controlled electric motor or a spring-driven 
escapement mechanism. In some specialized consumer 
services, switches which operate on a thermal princi- 
ple are used. In these the contact assembly is 
operated by the distortion of a thermal element when 
the latter has been carrying a designed current for a 
pre-determined period. 

An example of a motor-driven time switch unit is 


shown in Fig. 8.8. It is designed to actuate relays 
which, in turn, control the supply of alternating cur- 
rent to the heating elements of a power unit de-icing 
system (see p. 150). Signals to the relays are given in 
repeated time cycles which can be of short or long 
duration corresponding respectively to "fast" and 
"slow" selections made on the appropriate system 
control switch. 

gear ossembly 

Intermediate gear Yy 
assembly * 

ComshoM ossembly 


Fig 8.8 
Time switch unit 

The unit comprises an assembly of five cam and 
lever-actuated micro-switches driven by an a.c. motor 
through a reduction gearbox. 

The motor runs at constant speed and drives the 
camshaft at one revolution per 240 seconds. Two of 
the cams are of the three-lobcd type and they switch 
on two micro-switches three times during one revolu- 
tion, each "on" period corresponding to 20 seconds. 
Two other cams are of the single-lobed type and they 
switch on two associated micro-switches once during 
one revolution, the "on" periods in this case corre- 
sponding to 60 seconds. Thus the foregoing cam and 
micro-switch operations correspond respectively to 
"fast" and "slow" selections of power to the heating 
elements, which are accordingly heated for short or 
long periods. The fifth cam and its micro-switch con- 
stitute what is termed a "homing" control circuit, the 
purpose of which is to re-set the time switch after use 
so that it will always re-commence at the beginning of 
an operating cycle. 

When the "homing" micro-switch closes, it com- 
pletes an external relay circuit whose function is to 
continue operation of the motor whenever the de- 
icing system is switched off. On completion of the 
full revolution of the camshaft, the homing micro- 

switch is opened, thereby stopping the motor and 
resetting the timer for the next cycle of operation. 


Mercury switches are glass tubes into which stationary 
contacts, or electrodes, and a pool of loose mercury 
are hermetically sealed. Tilting the tube causes the 
mercury to flow in a direction to close or open a gap 
between the electrodes to "make" or "break" the 
circuit in which the switch is connected. 

The rapidity of "make" and "break" depends on 
the surface tension of the mercury rather than on 
externally applied forces. Thus, mercury switches are 
applied to systems in which the angular position of a 
component must be controlled within a narrow band 
of operation, and in which the mechanical force 
required to tilt a switch is very low. A typical appli- 
cation is in torque motor circuits of gyro horizons 
in which the gyros must be precessed to, and main- 
tained in, the vertical position. 

Mercury switches are essentially single-pole, single- 
throw devices but, as will be noted from Fig. 8.9, some 
variations in switching arrangements can be utilized. 


In many of the aircraft systems in which pressure 
measurement is involved, it is necessary that a warning 
be given of either low or high pressures which might 
constitute hazardous operating conditions. In some 
systems also, the frequency of operation may be such 
that the use of a pressure-measuring instrument is not 
justified since it is only necessary for some indication 
that an operating pressure has been attained for the 
period during which the system is in operation. To 
meet this requirement, pressure switches are installed 
in the relevant systems and are connected to warning 
or indicator lights located on the cockpit panels. 

A typical switch is illustrated in Fig. 8.10. It con- 
sists of a metal diaphragm bolted between the flanges 
of the two sections of the switch body. As may be 
seen, a chamber is formed on one side of the dia- 
phragm and is open to the pressure source. On the 
other side of the diaphragm a push rod, working 
through a sealed guide, bears against contacts fitted 
in a terminal block connected to the warning or indi- 
cator light assembly. The contacts may be arranged 
to "make" on either decreasing or increasing pressure, 
and their gap settings may be preadjusted in accord- 
ance with the pressures at which warning or indication 
is required. 

Pressure switches may also be applied to systems 


control i 


Fig 8.9 
Mercury switches 



Con"c* box 


Pressure plots 
Pressure inlet 

Fig 8.10 

Typical pressure switch unit 


requiring that warning or indication be given of 
changes in pressure with respect to a certain datum 
pressure; in other words, as a differential pressure 
warning device. The construction and operation are 
basically the same as the standard type, with the ex- 
ception that the diaphragm is subjected to a pressure 
on each side. 


Thermal switches are applied to systems in which a 
visual warning of excessive temperature conditions, 
automatic temperature control and automatic opera- 
tion of protection devices are required. Examples of 
such applications arc, respectively, overheating of a 
generator, control of valves in a thermal de-icing 
system and the automatic operation of fire extin- 

The principle most commonly adopted for thermal 
switch operation is the one based on the effects of 
differences of expansion between two metals, usually 
invar and steel. In some cases mercury contact 
switches may be employed. The general construction 
and operating details of some typical thermal switches 
are given in the following paragraphs. 

The switch illustrated in Fig. 8.1 1 is an example of 

a unit operating on the differential expansion prin- 
ciple, and utilized as an overheat warning device in a 
thermal de-icing system. The heat-sensitive element 
consists of a stainless steel tube with an invar rod 
supported internally and concentric with the tube. 
The tube is brazed to the rod at one end and at the 
other end it is brazed to an invar neck piece which is 
connected to the switch housing and serves as a guide 
for the rod. The switch element consists basically of 
a fixed contact and a spring-controlled movable 
contact connected in a warning light circuit. In the 
"cold" condition the length of the invar rod is such 
that it holds the contacts open. 

When the heat-sensitive element is subjected to a 
rising temperature the steel tube will increase its 
length whilst the original length of the invar rod 
will remain virtually unchanged. Since it is fixed at 
one end of the tube, it then will be drawn away from 
the switch element and after a pre-set movement 
corresponding to the set temperature, the movable 
contact will be displaced under the influence of its 
spring contact and the circuit completed to the 
warning lamp. When temperature conditions cool 
down the sensing element contracts and the switch 
re-sets itself. 

Snop aclion 

Terminal block 

Pivot pm 


Stainless steel 


Pivot pin 

Fig 8.1 1 

Overheat warning device 


Fig. 8.12 shows another example of a differential 
expansion switch employed as a fire detecting device. 
The heat-sensitive element is an alloy steel barrel 
containing a spring bow assembly of low coefficient 
of expansion. Each limb of the bow carries a silver- 
rhodium contact connected by fire-resistant cable to 
a terminal block located within a steel case. 

In the event of a fire or sufficient rise in tempera- 
ture at the switch location (a typical temperature is 
300°C) the barrel will expand and remove the com- 
pressive force from the bow assembly, permitting the 
contacts to close the circuit to its relevant warning 
lamp. When the temperature drops, the barrel con- 
tracts, thus compressing the bow assembly and re- 
opening the contacts. 


Mounting lug 

Fig 8.12 
Fire detector switch 


These switches are used in several types of aircraft as 
part of circuits required to give warning of whether or 
not passenger entrance doors, freight doors, etc. are 
fully closed and locked. Since they have no moving 
parts they offer certain advantages over micro-switches 
which are also applied to such warning circuits. 

A typical switch shown in Fig. 8.13 consists of two 
main components, one of which is an hermetically- 
sealed permanent magnet actuator, and the other a 
switch unit comprising two reeds, each having 
rhodium-plated contacts connected to the warning 
circuit. The two components are mounted in such a 
manner that when they contact each other, the field 
from the permanent magnet closes the reeds and con- 
tacts together, to complete a circuit to the "door 
closed" indicator. 


Relays are in effect, electromagnetic switching devices 

Switch unit 

Mognet actuator 

Fig 8.13 
Proximity switch 

by means of which one electrical circuit can be in- 
directly controlled by a change in the same or another 
electrical circuit. 

Various types of relay are in use, their construction, 
operation, power ratings, etc., being governed by their 
applications, which are also varied and numerous. In 
the basic form, however, a relay may be considered as 
being made up of two principal elements, one for 
sensing the electrical changes and for operating the 
relay mechanism, and the other for controlling the 
changes. The sensing and operating element is a 
solenoid and armature, and the controlling element 
is one or more pairs or contacts. 

As in the case of switches, relays are also designated 
by their "pole" and "throw" arrangements and these 
can range from the simple single-pole, single-throw 
type to complex multiple contact assemblies control- 
ling a variety of circuits and operated by the one 

In many applications the solenoid is energized 
directly from the aircraft power supply, while in 
others it may be energized by signals from an auto- 
matic device such as an amplifier in a cabin tempera- 
ture-control system, or a fire detector unit. When the 
solenoid coil is energized a magnetic field is set up 
and at a pre-determined voltage level (called the 
"pull-in" voltage) the armature is attracted to a pole 
piece against spring restraint, and actuates the contact 
assembly, this in turn cither completing or inter- 
rupting the circuit being controlled. When the solenoid 
coil circuit is interrupted at what is termed the "drop- 
out" voltage, the spring returns the armature and con- 
tact assembly to the inoperative condition. 

In addition to the contact assembly designations 


mentioned earlier, relays are also classified by the 
order of making and breaking of contacts, whether 
normally open ("NO") or normally closed ("NC") 
in the de-energized position, rating of the contacts in 
amperes and the voltage of the energizing supply. The 
design of a relay is dictated by the function it is re- 
quired to perform in a particular system or component, 
and as a result many types are available, making it 
difficult to group them neatly into specific classes. 
On a very broad basis, however, grouping is usually 
related to the basic form of construction, e.g. 
attracted core, attracted-armature, polarized armature, 
and "slugged", and the current-carrying ratings of the 
controlling element contacts, i.e. whether heavy-duty 
or light-duty. The descriptions given in the following 
paragraphs are therefore set out on this basis and the 
relays selected are typical and generally representative 
of applications to aircraft systems. 

The designation "heavy-duty" refers specifically to 
the amount of current to be carried by the contacts. 
These relays are therefore applied to circuits involving 
the use of heavy-duty motors which may take starting 
currents over a range from 100 A to 1 500 A, either 
short-term, as for starter motors for example, or con- 
tinuous operation. 

A relay of the type used for the control of a 
typical turbopropeller engine starter motor circuit is 
illustrated in Fig. 8.14. The contact assembly consists 
of a thick contact plate and two suitably insulated 
fixed contact studs connected to the main terminals. 

Coil assembly 

Carbon orcing contact 

Return soring 

Contact plale 

Spindle and 
_, „ , . collar assembly 

Fig 8.14 

Attracted core heavy-duty relay 

The contact plate is mounted on a supporting spindle 
and this also carries a soft inner core located inside 
the solenoid coil. The complete moving component 
is spring-loaded to hold the contact plate from the 
fixed contacts and to retain the core at the upper end 
of the coil. When the coil is energized the polarities 
of the magnetic fields established in the coil and core 
arc such that the core moves downwards against 
spring pressure, until movement is stopped by the 
contact plate bridging across the fixed contacts, thus 
completing the main circuit. Carbon contacts are 
provided to absorb the initial heavy current and 
thereby reduce arcing to a minimum before positive 
connection with the main contacts is made. 

A relay designed for use in a 28-volt d.c. circuit and 
having a contact rating of 3 A is shown in Fig. 8.15. 

Moving contact 


Fixed confoct 


Fig 8.15 

Attracted armature light-duty relay (sealed) 

The contacts are of a silver alloy and are actuated in 
the manner shown in the inset diagram, by a pivoted 
armature. In accordance with the practice adopted 
for many currently used relays, the principal elements 
are enclosed in an hermetically-sealed case filled with 
dry nitrogen and the connection in the circuit is made 
via a plug-in type base. Fig. 8.16 illustrates another 
example of attracted armature relay. This is of the 
unsealed type and is connected into the relevant cir- 
cuit by means of terminal screws in the base of the 


Core head 

Conlact spring 



nol bracket 
Restoring spring 

Fig 8.16 

Attracted armature relay (unsealed type) 

In certain specialized applications, the value of con- 
trol circuit currents and voltages may be only a few 
milliamps and millivolts, and therefore relays of excep- 
tional sensitivity are required. This requirement cannot 
always be met by relays which employ spring- 

controlled armatures, for although loading may be 
decreased to permit operation at a lower "pull-in" 
voltage, effective control of the contacts is decreased 
and there is a risk of contact flutter. A practical 
solution to this problem resulted in a relay in which 
the attraction and repulsion effects of magnetic 
forces are substituted for the conventional spring- 
control of the armature and contact assembly. Fig. 
8.17 shows, in diagrammatic form, the essential 
features and operating principle of such a relay. 

The armature is a permanent magnet and is 
pivoted between two sets of pole faces formed by a 
frame of high permeability material (usually mu- 
metal). It is lightly biased to one side to bring the 
contact assembly into the static condition as in Fig. 
8.17(a). The centre limb of the frame carries a low- 
inductance low-current winding which exerts a small 
magnetizing force on the frame when it is energized 
from a suitable source of direct current. With the 
armature in the static condition, the frame pole-faces 
acquire, by induction from the armature, the polarities 
shown, and the resulting forces of magnetic attraction 
retain the armature firmly in position. 

When a d.c. voltage is applied to the coil the frame 
becomes, in effect, the core of an electromagnet. The 
flux established in the core opposes and exceeds the 
flux due to the permanent magnet armature, and the 




Fig 8. 17 

Principle of a polarized armature relay 


frame pole-faces acquire the polarities shown in Fig. 
8.17(b). As the armature poles and frame pole-faces 
are now of like polarity, the armature is driven to the 
position shown in Fig. 8.17(c) by the forces of repul- 
sion. In this position it will be noted that poles and 
pole-faces are now of unlike polarity, and strong forces 
of attraction hold the armature and contact assembly 
in the operating condition. The fluxes derived from 
the coil and the armature act in the same direction to 
give a flux distribution as shown in Fig. 8.17(c). When 
the coil circuit supply is interrupted, the permanent 
magnet flux remains, but the force due to it is weaker 
than the armature bias force and so the armature and 
contacts are returned to the static condition (Fig. 


For some applications requirements arise for the use 
of relays which are slow to operate the contact assem- 
bly either at the stage when the armature is being 
attracted, or when it is being released. 

Some relays are therefore designed to meet these 
requirements, and they use a simple principle whereby 
the build-up or collapse of the main electromagnet 
flux is slowed down by a second and opposing mag- 
netizing force. This procedure is known as "slugging" 
and a relay to which it is applied is called a "slug" 
relay. The relay usually incorporates a ring of copper 
or other non-magnetic conducting material (the 
"slug") in the magnetic circuit of the relay, in such 
a way that changes in the operating flux which is 
linked with the slug originate the required opposing 
magnetic force. In some slug relays the required result 
is obtained by fitting an additional winding over the 
relay core and making provision for short-circuiting 
the winding, as required, by means of independent 
contacts provided in the main contact assemblies. 

Test Questions 

1. The number of circuits which can be completed 
through the poles of a switch is indicated by the 

(a) pole. 

(b) position. 

(c) throw. 

2. What do you understand by the term "position" 
in relation to toggle switches? 

3. To which circuits are (a) push-switches and (b) 
rotary switches normally applied? 

4. Describe the construction and operation of a 

5. What are the three main stages of movement of a 
micro-switch operating plunger? 

6. Describe the construction and operation of a 
mercury switch arranged to "break" a circuit. 

7. In a thermal switch employing steel and invar 
elements, actuation of the contacts under in- 
creasing temperature conditions is caused by: 

(a) expansion of the steel element only. 

(b) contraction of the invar element only. 

(c) expansion of the steel element causing dis- 
placement of the invar element. 

8. What are the principal ways in which relays may 
be classified? 

9. What do you understand by the terms "pull-in" 
voltage and "drop-out" voltage? 

10. Sketch a cross-section of a typical pressure switch; 
explain its operation. 

1 1 . What type of relay is required for a circuit in 
which control circuit current is of a very low value? 
Briefly describe the relay and its operation. 

12. (a) For what purpose are "slugged" relays used? 
(b) Describe the methods usually adopted for 

obtaining the slugging effects. 


Circuit Protection Devices 

In the event of a short circuit, an overload or other 
fault condition occurring in the circuit formed by 
cables and components of an electrical system, it is 
possible for extensive damage and failure to result. 
For example, if the excessive current flow caused by 
a short circuit at some section of a cable is left un- 
checked, the heat generated in the cable will continue 
to increase until something gives way. A portion of 
the cable may melt, thereby opening the circuit so 
that the only damage done would be to the cable 
involved. The probability exists, however, that much 
greater damage would result; the heat could char and 
bum the cable insulation and that of other cables 
forming a loom, and so causing more short circuits 
and setting the stage for an electrical fire. It is essen- 
tial therefore to provide devices in the network of 
power distribution to systems, and having the common 
purpose of protecting their circuits, cables and com- 
ponents. The devices normally employed are fuses, 
circuit breakers and current limiters. In addition, 
other devices are provided to serve as protection 
against such fault conditions as reverse current, 
overvoltage, undervoltage, overfrequency, under- 
frequency, phase imbalance, etc. These devices may 
generally be considered as part of main generating 
systems, and those associated with d.c. power genera- 
tion, in particular, are normally integrated with the 
generator control units. 


A fuse is a thermal device designed primarily to pro- 
tect the cables of a circuit against the flow of short- 
circuit and overload currents. In its basic form, a fuse 
consists of a low melting point fusible element or link, 
enclosed in a glass or ceramic casing which not only 
protects the element, but also localizes any flash which 
may occur when "fusing". The element is joined to 

end caps on the casing, the caps in turn, providing the 
connection of the element with the circuit it is 
designed to protect. Under short-circuit or overload 
current conditions, heating occurs, but before this 
can affect the circuit cables or other elements, the 
fusible element, which has a much lower current- 
carrying capacity, melts and interrupts the circuit. The 
materials most commonly used for the elements are 
tin, lead, alloy of tin and bismuth, silver or copper in 
either the pure or alloyed state. 

The construction and current ratings of fuses vary, 
to permit a suitable choice for specific electrical instal- 
lations and proper protection of individual circuits. 
Typical examples of fuses currently in use in light 
and heavy-duty circuits, are shown in Fig. 9.1(a>(b) 
respectively. The light-duty fuse is screwed into its 
holder (in some types a bayonet cap fitting is used) 
which is secured to the fuse panel by a fixing nut. 
The circuit cable is connected to terminals located in 
the holder, the terminals making contact with corre- 
sponding connections on the element cartridge. A 
small hole is drilled through the centre of the cap 
to permit the insertion of a fuse test probe. 

The heavy-duty or high rupturing capacity fuse 
(Fig. 9.1(b)) is designed for installation at main 
power distribution points (by means of mounting 
lugs and bolts). It consists of a tubular ceramic car- 
tridge within which a number of identical fuse 
elements in parallel are connected to end contacts. 
Fire-clay cement and metallic end caps effectively 
seal the ends of the cartridge, which is completely 
filled with a packing medium to damp down the 
explosive effect of the arc set up on rupture of the 
fusible elements. The material used for packing of 
the fuse illustrated is granular quartz; other materials 
suitable for this purpose are magnesite (magnesium 
oxide), kieselguhr, and calcium carbonate (chalk). 
When an overload current condition arises and each 


Clomp nul 




Fig 9.1 

Typical fuses 

(a) Light duty circuit fuse 

(b) High-rupturing capacity fuse 

element is close to fusing point, the element to go 
first immediately transfers its load to the remaining 
elements and they, now being well overloaded, fail in 
quick succession. 

In some transport aircraft, the fuseholders are of 
the self-indicating type incorporating a lamp and a 
resistor, connected in such a way that the lamp lights 
when the fusible element ruptures. 


Current limiters, as the name suggests, are designed to 
limit the current to some pre-determined amperage 
value. They are also thermal devices, but unlike or- 
dinary fuses they have a high melting point, so that 
their time/current characteristics permit them to carry 
a considerable overload current before rupturing. For 
this reason their application is confined to the pro- 
tection of heavy-duty power distribution circuits. 
A typical current limiter (manufactured under the 

name of "Airfuse") is illustrated in Fig. 9.2. It incor- 
porates a fusible element which is, in effect, a single 
strip of tinned copper, drilled and shaped at each end 
to form lug type connections, with the central portion 
"waisted" to the required width to form the fusing 
area. The central portion is enclosed by a rectangular 
ceramic housing, one side of which is furnished with 
an inspection window which, depending on the type, 
may be of glass or mica. 

Fir 9.2 
Typical current limiter ("Airfuse") 


These provide another form of protection particularly 
in d.c. circuits in which the initial current surge is 
very high, e.g. starter motor and inverter circuits, 
circuits containing highly-capacitive loads. When such 
circuits are switched on they impose current surges 
of such a magnitude as to lower the voltage of the 
complete system for a time period, the length of 
which is a function of the time response of the genera- 
ting and voltage regulating system. In order therefore 
to keep the current surges within limits, the starting 
sections of the appropriate circuits incorporate a 
resistance element which is automatically connected 
in series and then shorted out when the current has 
fallen to a safe value. 

Fig. 9.3 illustrates the application of a limiting 
resistor to a turbine engine starter motor circuit 
incorporating a time switch; the initial current flow 
may be as high as 1500 A. The resistor is shunted 
across the contacts of a shorting relay which is con- 
trolled by the time switch. When the starter push 
switch is operated, current from the busbar flows 
through the coil of the main starting relay, thus 
energizing it. Dosing of the relay contacts completes 
a circuit to the time switch motor, and also to the 
starter motor via the limiting resistor which thus 


reduces the peak current and initial starting torque 
of the motor. After a pre-determined time interval, 
which allows for a build-up of engine motoring 
speed, the torque load on the starter motor decreases 
and the time switch operates a set of contacts which 
complete a circuit to the shorting relay. As will be 
clear from Fig. 9.3, with the relay energized the 
current from the busbar passes direct to the starter 
motor, and the limiting resistor is shorted out. When 
ignition takes place and the engine reaches what is 
termed "self-sustaining speed", the power supply to 
the starter motor circuit is then switched off. 


•■, Master swilc» 

Starter motor 

Fig 9.3 

Application of a limiting resistor 


Circuit breakers, unlike fuses or current limiters, 
isolate faulted circuits and equipment by means of a 
mechanical trip device actuated by the heating of a 
bi- metallic element through which the current passes 
to a switch unit. We may therefore consider them as 
being a combined fuse and switch device. They are 
used for the protection of cables and components 
and, since they can be reset after clearance of a fault, 
they avoid some of the replacement problems associ- 
ated with fuses and current limiters. Furthermore, 
close tolerance trip time characteristics are possible, 
because the linkage between the bi-metal element 
and trip mechanism may be adjusted by the manu- 
facturer to suit the current ratings of the element. The 
mechanism is of the "trip-free" type, i.e. it will not 
allow the contacts of the switch unit to be held closed 
while a fault current exists in the circuit. 

The design and construction of circuit breakers 
varies, but in general they consist of three main 
assemblies; a bi-metal thermal element, a contact type 
switch unit and a mechanical latching mechanism. A 
push-pull button is also provided for manual resetting 
after thermal tripping has occurred, and for manual 
tripping when it is required to switch off the supply 
to the circuit of a system. The construction and opera- 
tion is illustrated schematically in Fig. 9.4. At (a) 

Contro! spring 



Fig 9.4 

Schematic diagram of circuit breaker operation 

(a) Closed 

(b) Tripped condition 

the circuit breaker is shown in its normal operating 
position; current passes through the switch unit con- 
tacts and the thermal element, which thus carries the 
full current supplied to the load being protected. At 
normal current values heat is produced in the thermal 
element, but is radiated away fairly quickly, and after 
an initial rise the temperature remains constant. It 
the current should exceed the normal operating value 
due to a short circuit, the temperature of the element 
begins to build up, and since metals comprising the 
thermal element have different coefficients of expan- 
sion, the element becomes distorted as indicated in 
Fig. 9.4(b). The distortion eventually becomes suf- 
ficient to release the latch mechanism and allows the 
control spring to open the switch unit contacts, thus 
isolating the load from the supply. At the same time, 
the push-pull button extends and in many types of 
circuit breaker a white band on the button is exposed 
to provide a visual indication of the tripped condition. 

The temperature rise and degree of distortion pro- 
duced in the thermal element are proportional to the 
value of the current and the time for which it is 
applied. The ambient temperature under which the 
circuit breaker operates also has an influence on cir- 


cuit breaker operation and this, together with operating 
current values and tripping times, is derived from 
characteristic curves supplied by the manufacturer. A 
set of curves for a typical 6 A circuit breaker is shown 
in Fig. 9.5. The current values are expressed as a 
percentage of the continuous rating of the circuit 
breaker, and the curves are plotted to cover specified 
tolerance bands of current and time for three amb- 
ient temperatures. If, for example, the breaker was 
operating at an ambient temperature of +57"C, then 
in say 30 seconds it would trip when the load current 


100 200 300 

Normol roted current (per cent) 


Fig 9.5 

Characteristic curves of a typical circuit breaker tripping times 

reached a value between 140 and 160 per cent of the 
normal rating, i.e. between 8-4 and 9-6 A. At an 
ambient temperature of +20°C it would trip in 30 
seconds at between 160 and 190 per cent of the 
normal rating (between 9-6 and 1 1-4 A) while at 
-40 C the load current would have to reach a value 

between 195 and 215 per cent of the normal rating 
(between 11-7 and 12-9 A) in order to trip in the same 
time interval. 

After a circuit breaker has tripped, the distorted 
element begins to cool down and reverts itself and 
the latch mechanism back to normal, and once the 
fault which caused tripping has been cleared, the cir- 
cuit can again be completed by pushing in the circuit 
breaker button. This "resetting" action closes the 
main contacts and re-engages the push-button with 
the latch mechanism. If it is required to isolate the 
power supply to a circuit due to a suspected fault, 
or during testing, a circuit breaker may be used as a 
switch simply by pulling out the button. In some 
designs a separate button is provided for this purpose. 

The external appearance of two typical single-pole, 
single-throw "trip-free" circuit breakers is illustrated 
in Fig. 9.6. The circuit breaker shown at (b) incor- 
porates a separate manual trip push button. A cover 
may sometimes be fitted to prevent inadvertent opera- 
tion of the button. 

In three-phase a.c. circuits, triple-pole circuit 
breakers are used, and their mechanisms arc so arranged 
that in the event of a fault current in any one or all 


Shakeproot washer 

Mounting nut 


White marker bond 

Terminal cover 

I b I 


Close button 
Trip button cover 

Trip button 

Pig 9.6 

Circuit breakers 

(a) Typical 

(b) Circuit breaker with a "manual trip" button 


three of the phases, all three poles will trip simul- 
taneously. Similar tripping will take place should an 
unbalanced phase condition develop as a result of a 
phase becoming "open-circuited". The three trip 
mechanisms actuate a common push-pull button. 

Protection Against Reverse Current 

In all types of electrical systems the current flow is, of 
course, from the power source to the distribution 
busbar system and finally to the power consuming 
equipment; the interconnection throughout being 
made by such automatic devices as voltage regulators 
and control units, and by manually controlled 
switches. Under fault conditions, however, it is pos- 
sible for the current flow to reverse direction, and as 
this would be of detriment to a circuit and associated 
equipment, it is therefore necessary to provide some 
automatic means of protection. In order to illustrate 
the fundamental principles we may consider two 
commonly used methods, namely the reverse current 
cut-out relay and the reverse current circuit breaker. 

A reverse current cut-out relay is used principally in 
a d.c. generating system either as a separate unit or 
as part of a voltage regulator, e.g. the one described 
on p. 9). The circuit arrangement, as applied to the 
generating system typical of several types of small 

aircraft, is shown in Fig. 9.7. The relay consists of 
two coils wound on a core and a spring-controlled 
armature and contact assembly. The shunt winding 
is made up of many turns of fine wire connected 
across the generator so that voltage is impressed on it at 
all times. The series winding, of a few turns of heavy 
wire, is in series with the main supply line and is 
designed to carry the entire line current. The winding 
is also connected to the contact assembly, which under 
static conditions is held in the open position by means 
of a spring. 

When the generator starts operating and the voltage 
builds up to a value which exceeds that of the battery, 
the shunt winding of the relay produces sufficient 
magnetism in the core to attract the armature and so 
close the contacts. Thus the relay acts as an auto- 
matic switch to connect the generator to the busbar, 
and also to the battery so that it is supplied with 
charging current. The field produced by the series 
winding aids the shunt-winding field in keeping the 
contacts firmly closed. 

When the generator is being shut down or, say, a 
failure in its output occurs, then the output falls 
below the battery voltage and there is a momentary 
discharge of current from the battery ; in other words, 
a condition of reverse current through the cut-out 
relay series winding is set up. As this also causes a 
reversal of its magnetic field, the shunt winding-field 
will be opposed, thereby reducing core magnetization 

Bjs co' 

— — teormol Current flow 
—•-- Reverse current flow 

Fig 9.7 

Reverse current cut-out operation 


until the armature spring opens the contacts. The 
generator is therefore switched to the "off-line" 
condition to protect it from damaging effects which 
would otherwise result from "motoring" current dis- 
charging from the battery. 

These circuit breakers are designed to protect power 
supply systems and associated circuits against fault 
currents of a magnitude greater than those at which 
cut-outs normally operate. Furthermore, they are 
designed to remain in a "locked-out" condition to 
ensure complete isolation of a circuit until a fault has 
been cleared. 

An example of a circuit breaker designed for use 
in a d.c. generating system is shown in Fig. 9.8. It 
consists of a magnetic unit, the field strength and 
direction of which are controlled by a single-turn coil 
connected between the generator positive output and 
the busbar via a main contact assembly. An auxiliary 
contact assembly is also provided for connection in 
series with the shunt-field winding of the generator. 

Rubber thioudcd 
wiling nonoie 

Mom let-u-c " 

Fig 9.8 
Reverse current circuit breaker 

The opening of both contact assemblies is controlled 
by a latching mechanism actuated by the magnet 
unit under heavy reverse current conditions. In com- 
mon with other circuit breakers, resetting after a 
tripping operation has to be done manually, and is 
accomplished by a lever which is also actuated by the 
latching mechanism. Visual indication of a tripped 
condition is provided by a coloured indicator flag 
which appears behind a window in the circuit breaker 
cover. Manual tripping of the unit is effected by a 
push-button adjacent to the resetting lever. 

Figure 9.9 is based on the circuit arrangement of a 
d.c. generating system used in a particular type of air- 
craft, and is an example of the application of a reverse 

current circuit breaker in conjunction with a cut-out 
relay. Unlike the circuit shown in Fig. 9.7, the relay 
controls the operation of a line contactor connected 
in series with the coil of the reverse current circuit 
breaker. Under normal current flow conditions closing 
of the relay energizes the line contactor, the heavy- 
duty contacts of which connect the generator output 
to the busbar via the coil and main contacts of the 
normally closed reverse current circuit breaker. The 
magnetic field set up by the current flow assists that 
of the magnet unit, thus maintaining the breaker con- 
tacts in the closed position. The generator shunt field 
circuit is supplied via the auxiliary contacts. 

When the generator is being shut down, or a failure 
of its output occurs, the reverse current resulting 
from the drop in output to a value below that of the 
battery flows through the circuit as indicated, and the 
cut-out relay is operated to de-energize the line con- 
tactor which takes the generator "offline". Under 
these conditions the reverse current circuit breaker 
will remain closed, since the current magnitude is 
much lower than that at which a specific type of 
breaker is normally rated (some typical ranges are 
200-250 A and 850-950 A). 

Let us consider now what would happen in the 
event of either the cut-out relay or the line contactor 
failing to open under the above low magnitude 
reverse current conditions, e.g. contacts have welded 
due to wear and excessive arcing. The reverse current 
would feed back to the generator, and in addition to 
its motoring effect on the generator, it would also 
reverse the generator field polarity. The reverse current 
passing through the circuit breaker coil would continue 
to increase in trying to overcome mechanical loads 
due to the engine and generator coupling, and so the 
increasing reverse field reduces the strength of the 
magnet unit. When the reverse current reaches the 
pre-set trip value of the circuit breaker, the field of 
the magnet unit is neutralized and repelled, causing 
the latch mechanism to release the main and auxiliary 
contacts to completely isolate the generator from the 
busbar. The breaker must be reset after the circuit 
fault has been cleared. 

Overvoltage protection 

Overvoltage is a condition which could arise in a 
generating system in the event of a fault in the field 
excitation circuit, e.g. internal grounding of the field 
windings or an open-circuit in the voltage regulator 
sensing lines. Devices are therefore necessary to pro- 



Main contacts 
Auxiliary contacts 

— Normal current flow 

— Reverse current flow 

Fig 9.9 

Reverse current circuit breaker operation 

tect consumer equipment against voltages higher than 
those at which they are normally designed to operate. 
The methods adopted vary between aircraft systems 
and also on whether they supply d.c. or a.c. An exam- 
ple of an overvoltage relay method applied to one 
type of d.c. system is shown in Fig. 9.10. 

The relay consists of a number of contacts con- 
nected in au essential circuits of the generator system, 
and mechanically coupled to a latching mechanism. 
This mechanism is electromagnetically controlled by 
a sensing coil and armature assembly, the coil being 
connected in the generator shunt-field circuit and in 
series with a resistor, the resistance of which de- 
creases as the current through it is increased. Under 
normal regulated voltage conditions, the sensing coil 
circuit resistance is high enough to prevent generator 
shunt-field current from releasing the relay latch 
mechanism, and so the contacts remain closed and 
the generator remains connected to the busbar. If, 

however, an open circuit occurs in the regulator vol- 
tage coil sensing line, shunt-field current increases 
and, because of the inverse characteristics of the relay 
sensing coil resistor, the electromagnetic field set up 
by the coil causes the latch mechanism to release all 
the relay contacts to the open position, thereby 
isolating the system from the busbar. After the fault 
has been cleared, the contacts are reset by depressing 
the push button. 

Figure 9.1 1 illustrates a method employed in a 
frequency-wild a.c. generating system, the full control 
of which is provided by magnetic amplifiers (see also 
Chapter 3). The output of the overvoltage protection 
magnetic amplifier is fed to a bridge rectifier and to 
the coil of a relay, via a feedback winding. The main 
contacts of the relay are connected in the normal d.c. 
supply switching circuit to the line contactor. 

Under normal voltage output conditions the im- 
pedance of the magnetic amplifier is such that its a.c. 




Line conloclor 


--— Overvoltoge sensing 

Fig 9.10 

Overvoltage protection d.c. generating system 

output, and the rectified a.c. through the relay coil, 
maintain the relay in the de-energized condition. When 
an overvoltage condition is produced the current 
through the relay coil increases to a pre-determined 
energizing value, and the opening of the relay contacts 
interrupts the d.c. supply to the line contactor, which 
then disconnects the generator from the busbar. At 
the same time, the main control unit interrupts the 
supply of self-excitation current to the generator, 

A.C. supply 

D.C. supply 

Fig 9.11 

Overvoltage protection a.c. generating system 

causing its a.c. output to collapse to zero. The relay 
resets itself and after the fault has been cleared the 
generator output may be restored and connected to 
the busbar by carrying out the normal starting cycle. 

Undervoltage Protection 

Undervoltage occurs in the course of operation when 
a generator is being shut down, and the flow of reverse 
current from the system to the generator is a normal 
indication of this condition. In a single d.c. generator 
system undervoltage protection is not essential since 
the reverse current is sensed and checked by the re- 
verse current cut-out. It is, however, essential in a 
multi-generator system with an equalizing method of 
load-sharing, and since a load-sharing circuit always 
acts to raise the voltage of a lagging generator, then 
an undervoltage protection circuit is integrated with 
that of load-sharing. A typical circuit normally com- 
prises a polarized relay which disconnects the load- 
sharing circuit and then allows the reverse current 
cut-out to disconnect the generator from the busbar. 
In an a.c. system, an undervoltage condition results in 
quadrature lagging current, or reactive power, which 


is the equivalent of reverse current. The protective 
function is performed by the reactive load-sharing 
circuit of a generating system, an example of which 
was described on pp. 43-45. 

Underfrequency and Overfrequency Protection 

Protection against these faults applies only to a.c. 
generating systems and is effected by the real load- 
sharing circuit of a generating system (see p. 42). 

Test Questions 

1. Sketch and describe the construction of two types 
of fuse suitable for aircraft use. Show how the 
construction affects their operating characteristics. 


2. What are the principal differences between a fuse 
and a current limiter as far as functions and appli- 
cations are concerned? 

3. State the function of a limiting resistor, and with 
the aid of a circuit diagram describe a typical 

4. A circuit breaker is a device for: 

(a) protecting an electrical circuit from current 

(b) collapsing the primary circuit of a magneto; 

(c) completing a circuit without being affected by 
current flow. 

5. With the aid of a sketch, describe the construction 
and explain the principle of operation, and 
characteristics, of a thermal circuit breaker. 

6. What is meant by the term "trip free" when 
applied to a thermal circuit breaker? 

7. Under what conditions would you say that it is 
permissible for a circuit breaker to be used as a 

8. What do you understand by the term "reverse 

9. Describe the operation of a reverse current cut- 

10. What is the function of a reverse current circuit 

1 1 . Briefly describe the operating principle of a reverse 
current circuit breaker. 

12. Describe a typical method of protecting a d.c. 
generating system against overvoltage. 


Power Utilization 

Our study of electrical systems thus far, has been 
concerned primarily with the fundamental principles 
of the methods by which power is produced and 
distributed, and also of the circuit protection methods 
generally adopted. This study, however, cannot be 
concluded without learning something of the various 
ways in which the power is utilized within aircraft. 
Utilization can extend over very wide areas depending 
as it does on the size and type of aircraft, and whether 
systems are employed which require full or only partial 
use of electrical power; therefore, in keeping with the 
theme of the book, we shall only concern ourselves 
with some typical aspects and applications. 

For the purpose of explanation, the subject is 
treated in this Chapter and in Chapter 1 1 respectively 
under two broad headings: (i) components used in 
conjunction with mechanical systems, e.g. an electric 
motor for operating a valve in a fuel system, or com- 
ponents used in electrical systems, e.g. lights; and (ii) 
systems which are principally all-clcctric, e.g. an engine 
starting and ignition system. 


A wide variety of components and systems ^depend 
upon mechanical energy furnished by motors and the 
numbers installed in any one type of aircraft depend 
on the extent to which electrical power is in fact 
utilized. A summary of some typical applications of 
motors is given in Table 10.1 

In most of the above applications the motors and 
mechanical sections of the equipment form inte- 
grated units. The power supply required for operation 
is 28 volts d.c. and/or 26-volts or 1 1 S-volts constant 
frequency a.c. and is applied almost without exception, 
by direct switching and without any special starting 
equipment. Many motors are required to operate only 
for a short time during a flight, and ratings between 

1 5 and 90 seconds are common. After operation at 
the rated load, a cooling period of as long as 10 to 
20 minutes may be necessary in some cases, e.g. a 
propeller feathering pump motor. 

Table 10.1 



Fuel "trimming"; Cargo door 

operation; Heat exchanger control- 

flap operation; Landing flap 


Control Valves 

Hot and cold air mixing for air- 

conditioning and thermal de-icing. 


Fuel delivery; Propeller feathering; 

De-icing fluid delivery. 

Flight Instruments 

Gyroscope operation; Servo 

and Control Systems 


Continuously-rated motors are often fan cooled 
and, in the case of fuel booster pumps which are of 
the immersed type, heat is transferred from the sealed 
motor casing to the fuel. Operating speeds are high 
and in cases where the energy from motors must be 
converted into mechanical movements, reduction 
gear-boxes are used as the transmission system. 

D.C. Motors 

The function and operating principle of d.c. motors 
is the reverse of generators, i.e. if an external supply 
is connected to the terminals it will produce motion 
of the armature thereby converting electrical energy 
into mechanical energy. This may be seen from Fig. 
10.1 which represents a motor in its simplest form, 
i.e. a single loop of wire " AB" arranged to rotate 
between the pole pieces of a magnet. The ends of the 
wire are connected to commutator segments which 
are contacted by brushes supplied with d.c. With 


Fig 10.1 

D.C. motor principle 

current flowing in the loop in the direction shown, 
magnetic fields are produced around the wire which 
interact with the main field and produce forces causing 
the loop to move in a clockwise direction. When the 
loop reaches a position at which the commutator 
reverses the polarity of the supply to the loop, the 
direction of current flow is also reversed, but due to 
the relative positions of the field around the wire and 
of the main field at that instant, the forces produced 
cause the loop to continue moving in a clockwise 
direction. This action continues so long as the power 
is supplied to the loop. 

As far as construction fundamentals are concerned, 
there is little difference between d.c. generators and 
motors; they both consist of the same essential parts, 
i.e. armature, field windings, commutator and brush- 
gear, the same methods of classifying according to 
various field excitation arrangements, and in the 
majority of motors the armature and field windings 
are supplied from a common power source, in other 
words they are self-excited. 

The application of a motor to a particular function 
is governed by two main characteristics; the speed 
characteristic and the torque characteristic. The 
former refers to the variation of speed with armature 
current which is determined by the back e.m.f., this, 
in its turn, being governed by the mechanical load on 
the motor. The torque characteristic is the relation- 
ship between the torque required to drive a given load 
and the armature current. 


There are three basic types of motors and as in the 
case of generators they are classified according to 
field excitation arrangements; series-wound, shunt- 
wound and compound-wound. These arrangements 
and certain other variations are adopted for a number 
of the functions listed in Table 1 0.1 and are illus- 
trated in Fig. 10.2. 



Fig 10.2 

Types of d.c. motor 

In series-wound motors, the field windings and the 
armature windings are connected in series with each 
other and the power supply. The currents flowing 
through both windings and the magnetic fields pro- 
duced are therefore the same. The windings are of 
low resistance, and so a scries motor is able to draw a 
large current when starting thereby eliminating 
building up the field strength quickly and giving the 
motor its principal advantages: high starting torque 
and good acceleration, with a rapid build-up of back 


e.m.f. induced in the armature to limit the current 
flow through the motor. 

The speed characteristic of a series wound motor 
is such that variations in mechanical load are accom- 
panied by substantial speed variations; a light load 
causing it to run at high speed and a heavy load 
causing it to run at low speed. 

The torque is proportional to the square of the 
armature current, and as an increase in load results in 
a reduction of the back e.m.f., then there is an increase 
in armature current and a rapid increase in driving 
torque. Thus the torque characteristic is such that a 
motor can be started on full load. 

In shunt-wound motors the field windings are 
arranged in the same manner as those of generators 
of this type, i.e. in parallel with the armature. The 
resistance of the winding is high and since it is con- 
nected directly across the power supply, the current 
through it is constant. The armature windings of some 
motors are of relatively high resistance and although 
their overall efficiency is low compared to the 
majority of shunt motors, they can be started by con- 
necting them directly to the supply source. For the 
starting of motors having low-resistance armature 
windings it is necessary for a variable resistance to be 
connected in series with the armature. At the start 
full resistance would be in circuit to limit the armature 
current to some predetermined value. As the speed 
builds up the armature current is reduced by the in- 
crease in back e.m.f. and then the resistance is pro- 
gressively reduced until, at normal speed of the motor, 
all resistance is out of the armature circuit. 

In operating from a "no-load" to a "full-load" 
condition the variation in speed of a motor with a 
low-resistance armature is small and the motor can 
be considered as having a constant-speed characteris- 
tic. In the case of a motor with a high-resistance 
armature there is a more noticeable difference in 
speed when operating over the above load conditions. 

The torque is proportional to the armature current 
until approaching full-load condition when the in- 
crease in armature reaction due to full-load current 
has a weakening effect. Starting torque is small since 
the field strength is slow to build up; thus, the torque 
characteristic is such that shunt-wound motors must 
be started on light or no load. 


For many applications it is necessary to utilize the 
principal characteristics of both series and shunt 
motors but without the effects of some of their 

normally undesirable features of operation. For 
example, a motor may be required to develop the 
high starting torque of a series type but without the 
tendency to race when load is removed. Other appli- 
cations may require a motor capable of reducing 
speed with increased load to an extent sufficient 
to prevent excessive power demand on the supply, 
while still retaining the smooth speed control and 
reliable "off-load" running characteristic of the 
shunt motor. These and other requirements can be 
met by what is termed compounding, or in other 
words, by combining both series and shunt field 
windings in the one machine. In most compound- 
wound motors the series and shunt windings are 
wound to give the same polarity on the pole faces so 
that the fields produced by each winding assist each 
other. This method of connection is known as cumu- 
lative compounding and there are three forms which 
may be used; normal, stabilized shunt and shunt 

In normal compounding a motor is biased towards 
the shunt-wound type, the shunt winding producing 
about 60 to 70 per cent of the total flux, the series 
winding producing the remainder. The desired charac- 
teristics of both series and shunt-wound motors are 

In the stabilized shunt form of compounding a 
motor is also biased towards the shunt-wound type 
but has a relatively minor series winding. The purpose 
of this winding is to overcome the tendency of a 
shunt motor to become unstable when running at or 
near its highest speed and then subjected to an in- 
crease in load. 

The shunt-limited motor is biased towards the 
series-wound type and has a minor shunt field winding 
incorporated in the field system. The purpose of the 
winding is to limit the maximum speed when running 
under "off-load" conditions while leaving the torque 
and general speed characteristics unaltered. Shunt 
limiting is applied only to the larger sizes of compound 
motors, typical examples being engine starter motors 
(see Fig. 10.3). 


In a number of applications involving motors it is 
required that the direction of motor rotation be 
reversed in order to perform a particular function, 
e.g. the opening and closing of a valve by an actuator. 
This is done by reversing the direction of current flow 
and magnetic field polarity, in either the field windings 
or the armature. 


Roller bearing Armature Terminal posls Ball bearing 


Oil seol Field coils Commutator Brush geor 

Fig 10.3 

Typical starter motor 

A method based on this principle, and one most 
commonly adopted in series-wound motors, is that in 
which the field winding is split into two electrically 
separate sections thereby establishing magnetic fields 
flowing in opposite directions. One of the two 
windings is used for each direction of rotation and is 
controlled by a single-pole double-throw switch. The 
circuit is shown in Fig. 10.4. When the switch is 
placed in the "Forward" position then current will 
flow in section "A" of the field winding and will 
establish a field in the iron core of appropriate polar- 
ity. Current also flows through the armature winding, 
the interaction of its field with that established by 
field winding section "A" causing the armature to 

rotate in the forward direction. When "Reverse" is 
selected on the control switch, section "A" is isolated 
and current flows through section "B" of the field 
winding in the opposite direction. The current flow 
through the armature is in the same direction as be- 
fore, but as the polarities of the iron core pole pieces 
are now reversed then the resultant interaction of 
fields causes the armature to run in the reverse direc- 
tion. Some split-field series motors are designed with 
two separate field windings on alternate poles. The 
armature in such a motor, a 4-pole reversible motor, 
rotates in one direction when current flows through 
the windings of one set of opposite pole pieces, and 
in the reverse direction when current flows through 
the other set of windings. 

The reversing of motors by interchanging the 
armature connections is also employed in certain 
applications, notably when the operating characteris- 
tics of compound machines are required. The circuit 
diagram illustrated in Fig. 10.5 is based on the 

Current flow-clockwise 
Current flow- anticlockwise 


Reversing reloy 


— i-o\ I .o4— — 

I Selector 
I switch 

Fig 10.4 
Split field motor circuit 

Fig 10.5 
Reversing of a compound motor 


arrangement adopted in a compound motor designed 
for the lowering and raising of an aircraft's landing 
flaps (see Fig. 10.6). Current flows to the armature 
winding via the contacts of a relay, since the current 
demands of the motor are fairly high. 

Bearing housing 


Front housing 


Broke drum assembly 

Output shoft 

, Clutch adjusting 
Armature ona \ grubscrew 
fon ossembly 

Yoke ona fiekJ 
COlls ossembly 

housing ossembly 

Fig 10.6 
Reversible compound motor 

Motor Actuators 

Motor actuators are self-contained units combining 
electrical and mechanical devices capable of exerting 
reversible linear thrust over short distances, or rever- 
sible low-speed turning effort. Actuators are thereby 
classified as either linear or rotary and may be 
powered by either d.c. or a.c. motors. In the majority 
of cases d.c. motors are of the split-field series-wound 


Linear actuators may vary in certain of their design 
and constructional features dependent upon the 
application, load requirements and the manufacturer 
responsible. In general, however, they consist of the 
motor which is coupled through reduction gearing to 
a lead screw which on being rotated extends or re- 
tracts a ram or plunger. Depending on the size of 
actuator, extension and retraction is achieved either 
by the action of a conventional screw thread or by 
what may be termed a "ball bearing thread". In the 
former case, the lead screw is threaded along its 
length with a square-form thread which mates with a 

corresponding thread in the hollow ram. With the 
motor in operation the rotary motion of the lead 
screw is thereby converted into linear motion of the 
ram, which is linked to the appropriate movable 

The ball bearing method provides a more efficient 
thread and is usually adopted in large actuators de- 
signed for operation against heavy loads. In this case, 
the conventional male and female thieads are re- 
placed by two semi-circular helical grooves, and the 
space between the grooves is filled with steel balls. 
As the lead screw rotates, the balls exert thrust on the 
ram, extending or retracting it as appropriate, and at 
the same time, a recirculating device ensures that the 
balls are fed continuously into the grooves. 

A typical linear actuator is shown in Fig. 10.7. 

Fig 10.7 

Linear actuator 


Rotary actuators are usually utilized in components 
the mechanical elements of which are required to be 
rotated at low speed or through limited angular travel. 
As in the case of linear actuators the drive from the 
motor is transmitted through reduction gearing, the 
output shaft of which is coupled directly to the rele- 
vant movable component, e.g. valve flap. Some 
typical examples of the application of rotary actuators 
are air-conditioning system spill valves and fuel cocks. 


The reduction gearing generally takes the form of 

multi-stage spur gear trains for small types of linear 

and rotary actuators, while in the larger types it is 

more usual for epicyclic gearing to be employed. The 

gear ratios vary between types of actuator and specific 




Both linear and rotary type actuators are equipped 
with limit switches to stop their respective motors 
when the operating ram or output shaft, as approp- 
riate, has reached the permissible limit of travel. The 
switches are of the micro type (see p. 103) and are 
usually operated by a cam driven by a shaft from the 
actuator gear-box. In some cases, limit switch con- 
tacts are also utilized to complete circuits to indicator 
lights or magnetic indicators. The interconnection of 
the switches is shown in Fig. 1 0.8, which is based on 
the circuit of a typical actuator-controlled valve 

In the "shut" position the cam operates the 
micro switch "A" so that it interrupts the "close" 
winding circuit of the motor and completes a circuit 
to the "shut" indicator. The contacts of the micro 
switch "B" are at that moment connected to the 
"open" winding of the motor so that when the con- 
trol switch is selected power is supplied to the winding. 
In running to the open position the cam causes micro 
switch "A" contacts to change over, thereby interrup- 
ting the indicator circuit and connecting the "close" 
winding so that the motor is always ready for opera- 
tion in either direction. As soon as the "open" position 

is reached the cam operates micro switch "B", the 
contacts of which then complete a circuit to the 
"open" indicator. 


The majority of actuators are fitted with electro- 
magnetic brakes to prevent over-travel when the motor 
is switched off. The design of brake system varies with 
the type and size of the actuator, but in all cases the 
brakes are spring-loaded to the "on" condition when 
the motor is de-energized, and the operating solenoids 
are connected in series with the armature so that the 
brakes are withdrawn immediately power is applied. 


Friction clutches, which are usually of the single-plate 
type or multi-plate type dependent on size of actuator, 
are incorporated in the transmission systems of actua- 
tors to proteci them against the effects of mechanical 

D.C. motors are not widely used in aircraft instru- 
ments, and in present-day systems they are usually con- 
fined to one or two types of turn-and-bank indicator 

28-V d.c. 

Magnetic indicators 
or lights 

Fig 10.8 

Limit switch operation 


to form the gyroscopic element. The motor armature 
together with a concentrically mounted outer rim 
forms the gyroscope rotor, the purpose of the rim 
being to increase the rotor mass and radius of gyration. 
The armature rotates inside a cylindrical two-pole 
permanent magnet stator secured to the gimbal ring. 
Current is fed to the brushes and commutator via 
flexible springs to permit gimbal ring movement. An 
essential requirement for operation of the instrument 
is that a constant rotor speed be maintained. This is 
achieved by a centrifugal cut-out type governor con- 
sisting of a fixed contact and a movable contact, 
normally held closed by an adjusting spring, and in 
series with the armature winding. A resistor is con- 
nected in parallel with the contacts. 

When the maximum speed is attained, centrifugal 
force acting on the moveable contact overcomes the 
spring restraint, causing the contacts to open. Current 
to the armature therefore passes through the resistor 
and so reduces rotor speed until it again reaches the 
nominal value. 

A.C. Motors 

In aircraft employing constant-frequency alternating 
current either as the primary or secondary source of 
electrical power, it is of course logical to utilize a.c. 
motors, and although they do not always serve as a 
complete substitute for d.c. machines, the advantages 
and special operating characteristics of certain types 
arc applied to a number of systems which rely upon 
mechanical energy from an electromotive power 

The a.c. motor most commonly used is the induc- 
tion type, and dependent upon the application may 
be designed for operation from a three-phase, two- 
phase or single-phase power supply. 


An induction motor derives its name from the fact 
that current produced in the rotating member, or 
rotor, is due to induced e.m.f. created by a rotating 
magnetic field set up by a.c. flowing in the windings 
of the stationary member or stator. Thus, inter- 
connection between the two members is solely mag- 
netic and as a result there is no necessity for a commit 
tator, slip rings and brushes. 

The rotor consists of a cylindrical laminated-iron 
core having a number of longitudinal bars of copper 
or aluminium evenly spaced around the circumference. 
These bars are joined at either end by copper or 

aluminium rings to form a composite structure com- 
monly called a "squirrel-cage". The stator consists of 
a number of ring-shaped laminations having slots 
formed on the inner surface and into which series- 
connected coil windings are placed. The number of 
windings and their disposition within the stator is 
directly related to the number of poles and phases of 
the power supply, e.g. more windings are required in 
a 4-pole motor than in a 2-pole motor both of which 
are to be operated from a 3-phase supply. 

The operating principle may be understood from 
Fig. 10.9, which represents a 2-pole 3-phase motor 
arrangement. Assuming that the relationship between 
phases (phase rotation) is as indicated, then al the 
instant "0", phases "A" and "C" are the only two 
carrying current and they set up magnetic fields which 
combine to form a resultant field acting downward 
through the rotor core. The field thus passes the bars 
of the squirrel-cage, and since they form a closed cir- 
cuit of low resistance the e.m.f. induced in the bars 
sets up a relatively large current flow in the direction 
indicated. As a result of the current flow magnetic 
fields are produced around the bars, each field inter- 
acting with the main field to produce torques on the 
rotor. This action is, in fact, the same as that which 
takes place in a d.c. motor and also a moving coil 
indicator. . 

— Direction of 

field . 




\ # 


@> / Q 

Phase 'a" Phase"B" Phase t" Phose "a" \/ D \ »_n 

■ ti /i < no 56 \. 

Resultant field Phase 8 

Al instonl 0" 
Phase "A" Phase ' 

A1 60" 
Phase "C" 


C0°\ _''l?0 o I80°\ /24C3O0"'\ 

Fig 10.9 
Induction motor principle 



Assuming now that the power supply frequency 
has advanced through 60 degrees, then phase "A" 
current falls to zero, phases "B" and "C" are the two 
now carrying current and so the resultant field pro- 
duced also advances through 60 degrees. In other 
words, the field starts rotating in synchronism with 
the frequency and establishes torques on the rotor 
squirrel-cage bars, thereby turning the rotor in the same 
direction as the rotating field in the stator. This 
action continues throughout the complete power 
supply cycle, the field making one complete revolu- 
tion. In the case of a 4-pole motor the field rotates 
only 180 degrees during a full cycle and a 6-pole 
motor only 90 degrees. 

As the speed of the rotor rises, there is a corre- 
sponding decrease of induced e.m.f. and torque until 
the latter balances the torques resulting from bearing 
friction, wind-resistance, etc., and the speed remains 
constant. Thus, the rotor never accelerates to the 
synchronous speed of the stator field; if it were to do 
so the bars would not be cut by the rotating field, 
there would be no induced e.m.f. or current flow, and 
no torque to maintain rotation. 

The synchronous speed of an induction motor is 
determined by the number of poles for which the 
stator is wound, and the frequency of the power 
supply, i.e. 

Synchronous Speed = f(Hz )x 60 

(rev/min) No. of pairs of poles 

The difference between the synchronous and rotor 
speeds, measured in r.p.m., is called the slip speed 
and the ratio of this speed to synchronous speed, 
expressed as a percentage, is called quite simply the 

As the name indicates these motors have only one 
stator winding, and as this alone cannot produce a 
rotating field to turn the rotor then some other 
method of self-starting is necessary. The method most 
commonly adopted is the one in which the main 
winding of the stator is split to produce a second 
starting winding. Thus we obtain what is usually 
called a split-phase motor, and by displacing the 
windings mutually at 90 electrical degrees, and arran- 
ging that the current in the starting winding either 
leads or lags on the main winding, a rotating field can 
be produced in the manner of a two-phase motor. 
After a motor has attained a certain percentage of its 
rated speed, the starting winding may be switched out 

of the circuit; it then continues to run as a single-phase 

The lagging or leading of currents in the windings 
is obtained by arranging that the ratio of inductive 
reactance to resistance of one winding differs con- 
siderably from that of the other winding. The varia- 
tions in ratio may be obtained by one of four methods, 
namely resistance starting, inductance starting, 
resistance/inductance starting or capacitance starting; 
the application of each method depends on the power 
output ratings of the particular motor. For example, 
horsepower ratings of capacitance starting motors are 
usually fractional and less than 2 h.p. 

The first three methods are used only during 
starting of a motor, because if both windings re- 
mained in circuit under running conditions, the per- 
formance would be adversely affected. Moreover, a 
motor is able to run as a single-phase machine once a 
certain speed has been reached. The starting winding 
circuit is normally disconnected by a centrifugal 
switch. The fourth method can be used for both 
starting and running, and with suitably rated capacitors 
the running performance of capacitor motors, as they 
are called, approaches that of two-phase motors. 

Figure 10. 10 illustrates the application of a 
squirrel-cage capacitor motor to an axial-flow blower 
designed for radio rack cooling or general air circu- 
lation. It utilizes two capacitors connected in parallel 
and operates from a 1 1 5-volts single-phase 400 Hz 
supply. The capacitive reactance of the capacitors is 
greater than the inductive reactance of the starting 
winding, and so the current through this winding 

lto:or trome ona siator 

Copaoror oox 


Piasnproof f ube 
Outer cosing 

Toil losing 

Con'OCl r-nq 

Sp.der mounting 

T ernvnal block 
To'i end bearing 

laier casing 

Drive end 

D-rve piaie bearing 

Fig 10.10 

Motor-driven blower 


thereby leads the supply voltage. The current in the 
running winding lags on the supply voltage and the 
phase difference causing field rotation is therefore the 
sum of the lag and lead angles. 

These find their greatest applications in systems 
requiring a servo control of synchronous devices, e.g. 
as servomotors in power follow-up synchro systems. 
The windings are also at 90 degrees to each other but, 
unlike the motors thus far described, they are con- 
nected to different voltage sources. One source is 
the main supply for the system and being of constant 
magnitude it serves as a reference voltage; the other 
source serves as a control voltage and is derived from 
a signal amplifier in such a way that it is variable in 
magnitude and its phase can either lead or lag the 
reference voltage, thereby controlling the speed and 
direction of rotation of the field and rotor. 


Hysteresis motors also consist of a stator and rotor 
assembly, but unlike other a.c. motors the operation 
is directly dependent on the magnetism induced in 
the rotors and on the hysteresis or lagging characteris- 
tics of the material (usually cobalt steel) from which 
they are made. 

A rotating field is produced by the stator and if 
the rotor is stationary, or turning at a speed less than 
the synchronous speed, every point on the rotor is 
subjected to successive magnetizing cycles. As the 
stator field reduces to zero during each cycle, a cer- 
tain amount of flux remains in the rotor material, and 
since it lags on the stator field it produces a torque at 
the rotor shaft which remains constant as the rotor 
accelerates up to the synchronous speed of the stator 
field. This latter feature is one of the principal advan- 
tages of hysteresis motors and for this reason they are 
chosen for such applications as autopilot servomotors, 
which produce mechanical movements of an aircraft's 
flight control surfaces. 

When the rotor reaches synchronous speed, it is 
no longer subjected to successive magnetizing cycles 
and in this condition it behaves as a permanent mag- 


lighting plays an important role in the operation of an 
aircraft and many of its systems, and in the main falls 
into two groups: external lighting and internal lighting. 

Some of the principal applications of lights within 
these groups are as follows: 

External Lighting 

(i) The marking of an aircraft's position by- 
means of navigation lights. 

(ii) Position marking by means of flashing 

(in) Forward illumination for landing and 

(iv) Illumination of wings and engine air 

intakes to check for icing, 
(v) Illumination to permit evacuation of 

passengers after an emergency landing. 

Internal Lighting 

(vi) Illumination of cockpit instruments and 

control panels, 
(vi) Illumination of passenger cabins and 

passenger information signs. 
(viii) Indication and warning of system operating 



The plan view of external lighting given in Fig. 10.1 1 
is based on the Boeing 747 and, although not all the 
lights shown would be standard on all other types of 
aircraft, it serves to illustrate the disposition of exter- 
nal lights generally. 


The requirements and characteristics of navigation 
lights are agreed on an international basis and are set 
out in the statutory Rules of the Air and Orders for 
Air Navigation and Air Traffic Control regulations. 
Briefly, these requirements are that every aircraft in 
flight or moving on the ground during the hours of 
darkness shall display: 

(a) A green light at or near the starboard wing tip, 
visible in the horizontal plane from a point 
directly ahead through an arc of 1 1 degrees to 

(b) A red light at or near the port wing tip, with a 
similar arc of visibility to port. 

(c) A white light visible from the rear of the aircraft 
in the horizontal plane through an arc of 140 
degrees. The conventional location of this light 
is in the aircraft's tail, but in certain cases, 
notably such aircraft as the Douglas DC- 10 and 
Lockheed 1011 "Tristar", white lights are 


Wing scon light - 

Green navigation light 

Red onticollision lights 
Top /\ 

Bottom / 


landing lighls 

egress lights 
LH and RH 

turnoff and 
taxi lighls 

Fig 10.11 

Disposition of external lighting 

Red navigation light 

mounted in the trailing edge sections of each 
wing tip. 

The above angular settings are indicated in Fig. 

The construction of the light fittings themselves 
varies in order to meet the installation requirements 
for different types of aircraft. In general, however, 
they consist of a filament type lamp, appropriate 
fitting and transparent coloured screen or cap. The 
screen is specially shaped and, together with the 
method of arranging the filament of the lamp, a sharp 
cut-off of light at the required angle of visibility is 
obtained. The electrical power required for the lights 
is normally 28 volts d.c. but in several current types 
of "all a.c." aircraft, the lights are supplied with 28 
volts a.c. via a step-down transformer. The operation 
of navigation lights, and their circuit arrangements, 
are factors which are dictated primarily by the regula- 
tions established for the flight operation of the types 
of aircraft concerned. Originally lights were required 
to give steady lighting conditions, but in order to im- 
prove the position marking function, subsequent 
developments provided for the lights to flash in a 
controlled sequence. However, following the adoption 
of flashing anti-collision beacons the requirement for 

flashing navigation lights was discontinued and the 
requirement for steady lighting conditions re- 
introduced to become the order of the day once more. 
It is possible, however, that flashing navigation lights 
may still be observed on occasions; these are installed 
in some aircraft below a certain weight category, 
registered before current requirements became effec- 
tive and thereby permitted alternative lighting 


Anti-collision lighting also fulfils a position marking 
function and, in conjunction with navigation lights 
giving steady lighting conditions, permits the position 
of an aircraft to be more readily determined. A 
lighting system may be of the type which emits a 
rotating beam of light, or of the strobe type from 
which short-duration flashes of high-intensity light 
are emitted. In some current types of aircraft both 
methods are used in combination, the strobe lighting 
forming what is termed "supplementary lighting". 

Rotating Beam Lights. These lights or beacons as 
they are often called, consist of a filament lamp unit 
and a motor, which in some cases drives a reflector 


and in others the lamp unit itself; the drive trans- 
mission system is usually of the gear and pinion type 
and of a specific reduction ratio. All components are 
contained within a mounting enclosed by a red glass 
cover. The power required for beacon operation is 
normally 28-volts d.c, but a number of types are 
designed for operation from an a.c. supply, the motor 
requiring 1 1 5 volts and the lamp unit 28 volts supp- 
lied via a step-down transformer. The motor speed 
and gear drive ratios of beacons are such that the 
reflector or lamp unit, as the case may be, is operated 
to establish a beam of light which rotates at a con- 
stant frequency. Typical speeds are 40-45 rev/min 
giving a frequency of 80—90 cycles per minute. There 
are several variations in the design of beacons, but 
the two types described here usefully serve as exam- 
ples of how the rotating reflector and rotating lamp 
techniques are applied. 

The beacon shown in Fig. 10.12 employs a V- 
shaped reflector which is rotated at about 45 rev/min 
by a d.c. motor, over and about the axis of a sealed 
beam lamp. One half of the reflector is flat and emits 
a narrow high-intensity beam of light near the hori- 
zontal, while the other half is curved to increase the 
up and down spread of its emitted beam to 30 degrees 
above and below the horizontal, and thereby reducing 
the light intensity. 

Figure 10.13 illustrates a beacon employing two 
filament lamps mounted in tandem and pivoted on 
their own axes. One half of each lamp forms a 
reflector, and the drive from the motor is so arranged 
that the lamps oscillate through 1 80 degrees, and as 
may be seen from the inset diagram, the light beams 
are 180 degrees apart at any instant. The power 
supply required for operation is a.c. 

Strobe Lighting. This type of lighting system is 
based on the principle of a capacitor-discharge flash 
tube. Depending on the size of the aircraft, strobe 
lighting may be installed in the wing tips to supple- 
ment the conventional red beacons, they may be used 
to function solely as beacons, or may be used in com- 
bination as a complete strobe type anti-collision high- 
intensity lighting system. 

The light unit takes the form of a quartz or glass 
tube filled with Xenon gas, and this is connected to a 
power supply unit made up essentially of a capacitor, 
and which converts input power of 28 volts d.c. or 
1 1 5 volts a.c. as the case may be, into a high d.c. out- 
put, usually 450 volts. The capacitor is charged to this 
voltage and periodically discharged between two 
electrodes in the Xenon-filled tube, the energy pro- 
ducing an effective high-intensity flash of light having 

Secj'-rg scew-s 



Re <iec lor -- 

Melol cover 

V— Red screen 







3- Pin connector 

Fig 10.12 

Rotating leflcctor beacon 


Fig 10.13 

Rotating lamp beacon 

a characteristic blue- white colour. A typical flashing 
frequency is 70 per minute. 

The unit shown in Fig. 10.14 is designed for wing 
tip mounting and consists of a housing containing the 
power supply circuitry, the tube, reflector and glass 
lens. When used as supplementary lighting or as a 

Fig 10.14 
Typical strobe light unit 

complete strobe anti-collision lighting system, three 
units are installed in trailing positions in each wing tip, 
and all lights are controlled in a flashing sequence by 
controllers and flasher timing units. 

As their names indicate these lamps provide essential 
illumination for the landing of an aircraft and for 
taxi-ing it to and from runways and terminal areas at 
night and at other times when visibility conditions are 
poor. Landing lamps arc so arranged that they illu- 
minate the runway immediately ahead of the aircraft 
from such positions as wing leading edges, front fuse- 
lage sections and nose landing gear structure. The 
lamps are of the sealed beam type and in some air- 
craft are mounted to direct beams of light at pre- 
determined and fixed angles. In other types of air- 
craft, the lamps may be extended to preselected 
angles, and retracted, by an electric motor and gear 
mechanism, or by a linear actuator. Micro-type limit 
switches are incorporated in the motor circuit and are 
actuated at the extreme limits of lamp travel to inter- 
rupt motor operation. 


A typical power rating for lamps is 600 watts, and 
depending on the design the power supply required 
for operation may be either d.c. or a.c. at 28 volts, 
the latter being derived from a 1 15-volts supply via a 
step-down transformer. In lamps of the retractable 
type which require a.c. for their operation, the motor 
is driven directly from the 1 1 5-volts supply. The 
supplies to the lamp and motor are controlled by 
separate switches on the appropriate control panel 
in the cockpit. The construction of a retractable type 
of lamp is shown in Fig. 10.15. 


Fig 10.15 

Typical landing lamp 

Taxi lamps are also of the sealed beam type and are 
located in the fuselage nose section, in most cases on 
the nose landing gear assembly. The power rating of 
the lamps is normally lower than that of landing lamps 
(250 watts is typical) and the supply required is 
either d.c. or a.c. at 28 volts. 

In certain cases the function of a taxi lamp is 
combined with that of a landing lamp. For example, 
in the unit illustrated in Fig. 10.15, the lamp has two 
filaments, one rated at 600 watts and the other at 400 
watts; both filaments provide the illumination for 

landing, while for taxi-ing only the 400 watt filament 
is used. 

In addition to taxi lamps some of the larger types 
of transport aircraft are equipped with lamps which 
direct beams of light to the sides of the runway (see 
Fig. 10.1 1). These are known as runway turn-off lights, 
their primary function being to illuminate the points 
along the runway at which an aircraft must turn to 
leave the runway after landing. 


Ice inspection or wing-scan lamps are fitted to most 
types of transport aircraft, to detect the formation of 
ice on the leading edges of wings and also at the air 
intakes of turbine engines. Lamps are also of the 
sealed beam d.c. or a.c. type and with power ratings 
varying from 60 watts to 250 watts depending on the 
lighting intensity required for a particular aircraft 
type. They are recessed into the sides of the fuselage 
and are preset to direct beams of light at the required 
angles. In some aircraft having rear-mounted engines 
lamps arc also recessed into the trailing edge sections 
of the wings. 

Internal Lighting 

The internal lighting of aircraft can be broadly divided 
into three categories: cockpit or operational lighting, 
passenger cabin lighting, and servicing lighting which 
includes galleys, toilet compartments, freight com- 
partments and equipment bays. 


The most important requirements for cockpit lighting 
are those necessary to ensure adequate illumination of 
all instruments, switches, controls, etc., and of the 
panels to which these items are fitted. The methods 
adopted to meet these requirements are of three main 

(i) integral lighting, i.e. one in which the light 

source is within each instrument; 
(ii) pillar and bridge lighting, in which a number 
of lights are positioned on panels to illumi- 
nate small adjacent areas, and to provide 
flood-lighting of individual instruments; 
(iii) flood-lighting, whereby lamps are positioned 
around the cockpit to flood-light an entire 
panel or general area. In some cases trans- 
illuminated panels are also used to permit 
engraved descriptions of various controls, 
notices and instructions to be readable under 

night conditions. A view of the Boeing 747 
cockpit under night lighting conditions is 
shown in Fig. 10.16. 

Colour of Cockpit Lighting. The choice of colour 
for the lighting of aircraft cockpits has always been 
the subject of numerous tests and studies and as far 
as the contributions to the safe and efficient operation 
of aircraft at night are concerned the choice has lain 
between red and white. Red lighting was introduced 
during the Second World War, its aims being twofold; 
firstly, to maintain a high degree of sensitivity to 
colour and amount of light (in other words dark 
adaptation) to enable pilots to search outside their 
aircraft for dim targets such as enemy aircraft and 
terrain during night operations; secondly to avoid 
lights being detected from outside and by the pilot 
of an enemy aircraft. The application of red lighting 
was also subsequently carried over to civil aircraft 


and was universally adopted as the principal lighting 
scheme, supplemented by a certain amount of white 
lighting. However, from continued tests and studies 
of the comparative merits of red and white lighting, 
it was generally concluded that maximum sensitivity 
to the amount of white light was not a requirement, 
that at the brightness levels adopted the use of low- 
intensity white light resulted in only a slightly 
inferior dark adaptation, and that the advantages 
of white light were very significant. White light is 
superior to red for several reasons, and these may be 
listed as follows: 

1. The amount of electrical power required is 
reduced since red filters absorbing about 80 
per cent of the light are eliminated. 

2. Heat dissipation problems are reduced. 

3. White light permits colour coding of displays, 
use of red warning flags and other similar indi- 

Fig 10.16 
Boeing 747 cockpit under night lighting conditions 


4. Contrasts between instrument displays and 
readability are improved. 

5. Eye fatigue is reduced. 

6. Better illumination is provided in thunderstorm 

There are a few disadvantages of course, but they 
are so outweighed by the advantages that white has 
become the recognized standard for instrument and 
panel lighting and is applied to many types of aircraft 
currently in service. 


The principal form of integral lighting for instruments 
is that known as wedge or front lighting; a form deriv- 
ing its name from the shape of the two portions which 
together make up the instrument cover glass. It relies 
for its operation upon the physical law that the angle 
at which light leaves a reflecting surface equals the 
angle at which it strikes that surface. The two wedges 
are mounted opposite to each other and with a 
narrow airspace separating them as shown in Fig. 
10.17. Light is introduced into wedge "A" from two 





Fig 10.17 

Wedge-type lighting 

6-volt lamps set into recesses in its wide end. A cer- 
tain amount of light passes directly through this 
wedge and on to the face of the dial while the re- 
mainder is reflected back into the wedge by its 
polished surfaces. The angle at which the light rays 
strike the wedge surfaces governs the amount of light 
reflected; the lower the angle, the more light is reflected. 

The double wedge mechanically changes the angle 
at which the light rays strike one of the reflecting 
surfaces of each wedge, thus distributing the light 
evenly across the dial and also limiting the amount of 
light given off by the instrument. Since the source of 
light is a radial one, the initial angle of some light 
rays with respect to the polished surfaces of wedge 
"A" is less than that of the others. The low-angle light 
rays progress further down the wedge before they 
leave and spread light across the entire dial, light 
escaping into wedge "B" is confronted with con- 
stantly decreasing angles, and this has the effect of 
trapping the light within the wedge and directing it to 
its wide end. Absorption of light reflected into the 
wide end of wedge "B" is ensured by painting its 
outer part black. 

Pillar lighting, so called after the method of construc- 
tion and attachment of the lamp, provides illumina- 
tion for individual instruments and controls on the 
various cockpit panels. A typical assembly, shown 
in Fig. 10.18, consists of a miniature centre-contact 
filament lamp inside a housing, which is a push fit 
into the body of the assembly. The body is threaded 
externally for attachment to the panel and has a hole 
running through its length to accommodate a cable 
which connects the positive supply to the centre con- 
tact. The circuit through the lamp is completed by a 
ground tag connected to the negative cable. 

Light is distributed through a filter and an aperture 
in the lamp housing. The shape of the aperture dis- 
tributes a sector of light which extends downwards 
over an arc of approximately 90 degrees to a depth 
slightly less than 2 in. from the mounting point. 

The bridge-type of lighting (Fig. 10.18(b)) is a 
multi-lamp development of the individual pillar lamp 
already described. Two or four lamps are fitted to a 
bridge structure designed to fit over a variety of the 
standardized instrument cases. The bridge fitting is 
composed of two Light alloy pressings secured to- 
gether by rivets and spacers, and carrying the requisite 
number of centre contact assemblies above which the 
lamp housings are mounted. Wiring arrangements pro- 




Fig 10.18 
Pillar and bridge lighting 

vide for two separate supplies to the lamps thus 
ensuring that total loss of illumination cannot occur 
as a result of failure of one circuit. 

The principle of trans-illumination is shown in Fig. 
10.19 and is based on the Thorn "Hasteck" system. 
A relatively thick sheet of acrylic plastic is faced on 
both its main surfaces by a thin sheet of translucent 
white plastic. Over this is laid a second thin sheet of 
black or grey opaque plastic and the whole assembly 






Fig 10.19 

Trans-illuminated panel 

is then bonded together to form a homogeneous 
panel. Cut-outs, corresponding to the locations of 
instruments to be mounted on the instrument or 
control panel, are made in the plastic panel which is 
fitted as an overlay to the appropriate cockpit panel. 
In addition, operational data, instructions, switch 
identifications, etc., are made directly on the panel by 
engraving through the outer layer without piercing the 
white layer. 

Miniature lamps ("pea" lamps) are embedded in the 
panel at the various positions required and they 
transmit light through the panel to the inside edges of 
the cut-outs, thereby spreading light evenly over the 
dials of the instruments, and also to back-illuminate 
the operational data, instructions, etc. 


Flood-lighting is used for the general illumination of 
instruments, control panels, pedestals, side consoles 
and areas of cockpit floors. The lights usually take 
the form of incandescent lamp units and fluorescent 
tube units and depending on the type of aircraft, both 
forms may be used in combination. 


This form of lighting is employed in a number of air- 


craft as passenger information signs and also, in some 
cases, for the illumination of instrument dials and 
selective positions of valves or switches. An electro- 
luminescent light consists of a thin laminate structure 
in which a layer of phosphor is sandwiched between 
two electrodes, one of which is transparent. The light 
requires a.c. for its operation, and when this is applied 
to the elctrodes the phosphor particles luminesce, 
i.e. visible light is emitted through the transparent 
electrode. The luminescent intensity depends on the 
voltage and frequency of the a.c. supply. The area of 
the phosphor layer which becomes "electrolumines- 
cent" when the current is applied is that actually 
sandwiched between the electrodes; consequently if 
the back electrode is shaped to the form of a letter 
or a figure the pattern of light emitted through the 
transparent electrode is an image of the back electrode. 

The extent to which lighting is used in a passenger 
cabin depends on the size of a cabin and largely on 
the interior decor adopted for the type of aircraft; 
thus, it can vary from a small number of roof-mounted 
incandescent lamp fittings to a large number of fluores- 
cent fittings located in ceilings and hat racks so as to 
give concealed, pleasing and functional lighting effects. 
The power supplies required are d.c. or a.c. as appro- 
priate, and in all commercial passenger transport air- 
craft the lights are controlled from panels at cabin 
attendant stations. In addition to main cabin lighting, 
lights are also provided for passenger service panels 
(see p. 153) and are required for the illumination of 
essential passenger information signs, e.g. "Fasten 
Seat Belts" and "Return to Cabin". The lights for 
these signs may be of the incandescent type or, in a 
number of aircraft, of the electroluminescent type 
described earlier. They are controlled by switches on 
a cockpit overhead panel. 


An essentia] requirement concerning lighting is that 
adequate illumination of the cockpit and the various 
sections of the cabin, exits, escape hatches, chutes 
etc., must be provided under emergency conditions, 
e.g. a crash-landing at night. The illumination is nor- 
mally at a lower level than that provided by the stan- 
dard lighting systems, since the light units are directly 
powered from an emergency battery or batteries or 
direct from the aircraft battery in some cases. The 
batteries are normally of the nickel-cadmium type 

although in some aircraft silver-zinc batteries are 

Under normal operating conditions of the aircraft, 
an emergency battery is maintained in a fully-charged 
condition by a trickle charge from the aircraft's main 
busbar system. 

Primary control of the lights is by means of a 
switch on a cockpit overhead panel, but in a number 
of aircraft types, a secondary control by means of 
inertia switches is also adopted. 

Test Questions 

1. Define the characteristics which govern the appli- 
cation of a d.c. motor to a particular function. 

2. What are the principal characteristics of a shunt- 
wound and series-wound motor? 

3. When the r.p.m. of a shunt-wound motor increases 
the current drawn by it: 

(a) decreases. 

(b) remains the same. 

(c) increases. 

4. Draw a circuit diagram of the motor to be applied 
to a system where high starting torque and steady 
"off-load" running is required. 

5. What is meant by the term "shunt limiting"? 

6. With the aid of a circuit diagram explain the 
operation of a motor required for simple reversing 

7. Actuator motors are prevented from over-running 
their limits of travel by means of: 

(a) manually controlled switches. 

(b) electromagnetic brakes. 

(c) cam-operated limit switches. 

8. Describe how the speed of a d.c. operated turn- 
and-bank indicator is maintained constant. 

9. (a) Explain how a three-phase rotating magnetic 

field is produced in an induction motor, 
(b) Why does the rotor run at a speed slightly 
less than that of the rotating field? 


10. In an a.c. motor, the difference between syn- 
chronous speed and the speed of the rotor is 

(a) the motor loss speed. 

(b) the brake speed. 

(c) the slip speed. 

1 1 . What is the formula for determining the syn- 
chronous speed of an induction motor? 

12. In terms of the amount of field rotation relative 
to one cycle of the power supply, what are the 


differences between 2-pole, 4-pole and 6-pole 

13. Describe how a rotating magnetic field is pro- 
duced in a single-phase induction motor. 

14. Describe the operation of a hysteresis motor and 
state one of its applications. 

15. A typical frequency of anti-collision light beam 
rotation is: 

(a) 40—45 cycles per minute. 

(b) 80—90 cycles per second. 

(c) 80-90 cycles per minute. 

1 6. Why are the two surfaces of a V-shaped reflector 
arranged differently from each other? 

17. What are the principal functions of a strobe lighting 

18. Describe the operating principle of a strobe lighting 

19. Describe one of the methods of illuminating instru- 
ment dials. 


Power Utilization 

Engine Starting Systems 

Throughout the development of aircraft engines a 
number of methods of starting them have been used 
and the prime movers involved have varied from a 
mechanic manually swinging a propeller, to electric 
motors and electric control of sophisticated turbo- 
starter units. Although there are still one or two types 
of light aircraft in service requiring the manual 
swinging technique, the most widely adopted starting 
method for reciprocating engines utilizes electric 
motors, while for the starting of gas turbine engines 
either electric motors or turbo-starter units may be 
utilized as the prime movers. 

In basic form, these systems consist of a motor, an 
engaging gear, a relay and a starter switch; in some 
systems a clutch mechanism is also incorporated in 
the engaging gear mechanism. The motors employed 
may be of the plain series-field type or may be com- 
pounded with a strong series bias (see pp. 121 and 122). 

Fig. 11.1 shows the interconnection of the principal 
electrical components typical of those required for 
the starting of reciprocating engines installed in many 
types of light aircraft. When the starter switch is closed, 
direct current from the battery and busbar energizes 
the starter relay, the closed contacts of which connect 
the motor to the battery. The relay contacts are of 
the heavy-duty type to carry the high current drawn by 
the motor during the period of cranking over the 

The method of engaging a motor with an engine 
varies according to the particular engine design. For 
most types of light aircraft engines, a pinion is engaged 
with a starter gear ring secured to the engine crank- 
shaft in a manner similar to that employed for starting 
automobile engines. When the engine starts, it over- 
runs the starter motor and the pinion gets "kicked 

out" of engagement. In other versions used for 
starting more powerful engines, a jaw engages with a 
similar member on the engine and the drive is trans- 
mitted via a clutch and reduction gear train in the 
starter motor and in an accessories gearbox in the 

The gear ratio between a starter motor and a re- 
ciprocating engine is such that it provides a low crank- 
ing speed of the engine; a typical reduction ratio is 
about 100 : 1. Cranking speed is not critical because of 
the fuel priming provisions made in the starting drill, and 
also because there is a good stream of sparks available 
at the plug points for the power stroke. Thus, once 
the engine has "fired" and gets away under its own 
power further assistance from the starter motor is 
rendered unnecessary. Although the moment of 
inertia of an engine's moving parts is comparatively 
light during cranking, a starter motor has to overcome 
some heavy frictional loads, i.e. loads of pistons and 
bearings, and also loads due to compression. 

Compared with a reciprocating engine, the starting of 
a turbine engine represents a relatively severe duty 
for the starter motor. This stems mainly from the 
starting principle involved and also from the construc- 
tion of the rotating assembly, e.g. whether the com- 
pressor and turbine are on a single shaft (single-spool 
engine) or whether high-pressure compressor turbine 
assemblies and low-pressure compressor turbine 
assemblies on separate shafts are employed (two- 
spool engine). Another factor also is whether the 
compressor and turbine assembly is designed to drive 
a propeller. In general, turbine engines have a high 
moment of inertia, and since it is a requirement that 
starting shall be effected as quickly as possible, then 
high gear ratios and therefore high cranking speeds 
are necessary. 







/ Starter 
J switch 




Fig 11.1 

Simple engine starting system 

The process of starting a turbine engine involves 
the provision of an adequate and continuous volume 
of air to the combustion system, effective atomiza- 
tion of fuel at the burners of the combustion system, 
and the initiation of combustion in the combustion 
chambers. To provide the necessary volume of air 
the starter motor must be capable of developing 
sufficient power to accelerate the compressor smoothly 
and gently from a static condition to a fairly high 
speed. At some stage in the cranking operation, fuel 
is injected into the combustion system and the fuel/ 
air mixture is ignited, i.e. the engine "fires" or "lights- 
up" as it is more usually stated in turbine engine 
terms. Unlike reciprocating engine starting, however, 
the starter motor does not disengage at this point but, 
assisted by the engine, continues to accelerate it up 
to a speed at which the engine alone is capable of 
maintaining rotation. This is known as the self- 
sustaining speed of the engine. Eventually a condition 
is reached where the starter motor is no longer 
required and its torque, and the current consumed, 
start decreasing fairly rapidly. Its speed will tend to 
increase, but this is limited by the retarding torque 

provided by the shunt field when there is no longer a 
load on the motor (see also p. 122). Depending on the 
type of starter system, the power supply to the 
starter motor is interrupted automatically either by 
the decrease in current causing the starter relay to de- 
energjze, or by the opening of contacts in a time 
switch unit. 

Fig. 1 1 .2 illustrates the circuit diagram of a system 
based on that employed in a current type of twin 
turbopropeller aircraft for the starting of its engines. 
The starter motor is a 28-volts d.c. four-pole 
compound-wound machine having a torque output of 
16-5 lbf.ft(22-37 Newton metre) at a speed of 3800 
rev/min and a time rating of 90 seconds. It drives the 
engine through a clutch, pawl mechanism and reduc- 
tion gear. The clutch is held in the driving position 
until the engine has accelerated above the starter 
motor speed and until the centrifugal force acting 
on the pawl mechanism is sufficient to release the 
pawls. The starter motor is disengaged by the action 
of an overspeed relay. 

When the master switch is set to the "start" posi- 
tion, and the starter push switch is depressed, direct 



To ignition system 

Fig 11.2 

Basic circuit of a turboprop engine starting system 

current flows through the coil of the main starter 
relay thereby energizing it. At the same time current 
also flows to contact "1" of the overspeed relay. The 
closing of the heavy-duty contacts "A" and "B" of 
the starter relay completes a circuit from the main 
busbar to the starter motor via the coil of the over- 
speed relay, which on being energized, allows current 
to flow across its contacts to the coil of the push 
switch thereby holding this switch closed. During 
initial stages of starting the current drawn by the 
starter motor is high, and as this is carried by the 
coil of the overspeed relay continued cranking of 
the engine is assured. As the engine accelerates, the 
starter motor draws less current until, at a value pre- 
determined by the speed at which the engine becomes 
self-sustaining, the overspeed relay is de-energised, this 
in turn de-energizing the starter switch and main 
starter relay. The overspeed relay therefore prevents 
the starter motor from overspeeding by ensuring that 
the power supply is disconnected before the starter 
drive is disengaged from the engine. 

The purpose of the "blow out" position of the 
master switch is to permit the engine to be cranked 
over in order to blow out unburnt fuel resulting from 
an unsuccessful start or "light up". When the position 
is selected, the circuit is operated in a similar manner 
to normal starting except that the starter switch must 
be pulled to the "off position after the motor has 

been running for 30 seconds. The reason for this is 
that since the ignition system is isolated, the starter 
motor is still heavily loaded and so the current 
through the overspeed relay remains too high for the 
relay to de-energize of its own accord. 

With the development of more powerful turbine en- 
gines ever-increasing power output from starter sys- 
tems was required for effective starting action. As 
far as electrical methods of starting were concerned 
this presented increasingly difficult problems associ- 
ated notably with high current demand, increased 
size and weight of motors and cables. These problems 
therefore led to the discontinuance of electric motors 
for the starting of powerful engines, and their func- 
tions were taken over by turbo-starter systems 
requiring a simpler control circuit consuming only a 
few amperes. 

There are three principal types of turbo-starter 
systems; air, cartridge and monofuel, the application 
of each being governed largely by the operational role 
of the aircraft, i.e. civil or military. The basic principle 
is the same for each system, that is, a gas is made to 
impinge on the blades of a turbine rotor within the 
starter unit, thereby producing the power required to 
turn the engine shaft via an appropriate form of 


The gas may be (i) compressed air from either an 
external supply unit, an A.P.U. in the aircraft or the 
compressor of a running engine; (ii) the cordite dis- 
charge from an electrically fired cartridge or (hi) the 
result of igniting a monofuel, in other words a fuel 
which burns freely without an oxidant such as air; 
a typical fuel is iso-propylnitrate. 

The electrical control circuits normally require d.c. 
for their operation, their function being to energize 
solenoid-operated air control valves, to fire cartridge 
units and to energize a fuel pump motor and ignition 
systems as appropriate to the type of turbo-starter 

Several types of turbine-powered aircraft are 
equipped with starter systems which utilize a prime 
mover having the dual function of engine starting and 
of supplying power to the aircraft's electrical system. 
Starter-generator units are basically compound-wound 
machines with compensating windings and interpoles, 
and are permanently coupled with the appropriate 
engine via a drive shaft and gear train. For starting 
purposes, the unit functions as a fully compounded 
motor, the shunt winding being supplied with current 
via a field changeover relay. When the engine reaches 
self-sustaining speed and the starter motor circuit is 
isolated from the power supply, the changeover relay 
is also automatically de-energized and its contacts 
connect the shunt-field winding to a voltage regulator. 
The relay contacts also permit d.c. to flow through 
the shunt winding to provide initial excitation of the 
field. Thus, the machine functions as a conventional 
d.c. generator, its output being connected to the bus- 
bar on reaching the regulated level. 

Ignition Systems 

All types of aircraft engines are dependent on electrical 
ignition systems. In reciprocating-type engines, the 
charges of fuel vapour and air which are induced and 
compressed in the cylinders, arc ignited through the 
medium of sparks produced by electric discharges 
across the gaps between the electrodes of a spark plug 
fitted in each cylinder, and a continuous series of 
high-voltage electrical impulses, separated by intervals 
which are related to engine speed, must be made 
available to each of the plugs throughout the period 
the engine is running. A basically similar electrical 
ignition system is also used to initiate combustion of 
the fuel/air mixture in the combustion chambers of 

gas turbine engines. It is, however, of much simpler 
form for the reasons that impulse intervals are not 
related to engine speed, and as combustion is con- 
tinuous after "light up", the ignition system is only 
required during the starting period. 

Reciprocating-type engine ignition systems fall into 
one or other of two main categories; coil ignition and 
magneto ignition. The former derives its power from 
an external source, e.g. the main power supply, while 
the magneto is a self-contained unit driven by the 
engine and supplying power from its own generator. 
In aircraft engine applications, magneto ignition is 
the system most commonly adopted. 

Magneto ignition systems, which operate on the 
principles of electromagnetic induction, are classified 
as either high tension or low tension, and they consist 
of the principal components shown schematically in 
Fig. 1 1.3. Most of these components are contained 
within the magneto, which is basically a combination 
of permanent-magnet a.c. generator and auto- 

The high tension system is the one most widely 
used, and the requisite alternating fluxes and voltages 
are induced either by rotating the transformer 
windings between the poles of a permanent magnet, 
by rotating the magnet between fixed transformer 
windings or by rotating soft-iron inductor bars be- 
tween fixed permanent magnet and transformer 
windings. These arrangements, respectively, permit 
further classification of magnetos as (i) rotating 
armature, (ii) rotating magnet and (iii) polar inductor. 
The rotating portion of a magneto is driven by the 
engine through a coupling and an accessory gear drive 
shaft. As the windings are cut by the alternating 
magnetic flux from the appropriate source, a low 
voltage is induced in the primary winding to produce 
a current and flux of a strength directly proportional 
to the rate at which the main flux is cut. At this point 
the primary circuit is broken by the contact breaker, 
the contacts, or points, of which are opened by a 
cam driven by the rotating assembly. The primary flux 
therefore collapses about the secondary winding, which 
produces a high voltage output. The output is, how- 
ever, not sufficient to produce the required discharge 
at the spark plugs and it is necessary to speed up the 
rate of flux collapse. This is effected by connecting a 
capacitor across the contact breaker so that the capaci- 
tor is shorted out when the breaker points are closed 
and is charged by primary winding current when the 


Secondory Prima 

^ ndin q 'winding 



Fig 11.3 

Magneto ignition system 

points are open. When the potential difference 
across the capacitor reaches the point whereby it dis- 
charges itself, the correspondingly high current flows 
through the primary winding in the reverse direction 
and thereby rapidly suppresses the primary flux to 
produce the required higher secondary output vol- 
tage. In addition to this function, the capacitor also 
prevents arcing between the contact breaker points 
as they begin to open, thereby preventing rapid 
deterioration of the points. 

The secondary winding output is supplied to the 
distributor, the purpose of which is to ensure that the 
high voltage impulses are conducted to the sparking 
plugs in accordance with the order in which combus- 
tion must take place in each cylinder, i.e. the "firing" 
order of the engine. A distributor consists of two 
main parts, a rotor made up of an insulating and a 
conducting material, and a block of insulating material 
containing conducting segments corresponding in 
number to the number of cylinders on the engine. 
The conducting segments are located circumferentially 
around the distributor block in the desired firing 
order, so that as the rotor turns a circuit is com- 
pleted to a sparking plug each time there is alignment 
between the rotor and a segment. 

Distributors usually form part of magnetos, and the 
rotors are rotated at the required speed by a gear 
driven from the main magneto shaft. In some cases, 
however, distributors may be separate units driven 
by an engine gear train and drive shaft. To prevent 
ionization, and to minimize "flashover", the distribu- 
tor casing is vented to atmosphere, and in many types 
of magnet a flameproof wire mesh screen is provided 
to prevent combustion of any flammable vapours 
round the engine. 

Ignition of the combustible mixture is required in each 
cylinder once in every two revolutions of the engine 
crankshaft, and as a result there must be a definitie 
relationship between such factors as the number of 
sparks produced by a magneto and the speeds of the 
magneto, distributor and engine. Magneto speed may 
be calculated from the relation: 

number of cylinders 

2 x magneto sparks per rev. 

A rotating armature magneto, which is normally 
only used on engines having up to six cylinders, produces 
two sparks per rev. Thus, assuming that one is fitted 


to a four-cylinder engine then it must be driven at the 
same speed as the engine. A rotating magnet or polar 
inductor magneto produces four sparks per rev and 
is normally used on engines having more than six 
cylinders. Thus, for a twelve-cylinder engine the 
magneto must be driven at one and a half times the 
engine speed. Distributor rotors are driven at half 
engine speed irrespective of magneto speed. 

As mentioned earlier, during starting, a piston engine 
is cranked over at very low speeds, and as a result its 
magnetos are driven much too slowly for the e.m.f. 
induced in the primary winding to produce a spark of 
adequate energy-content at the instant the contact 
breaker points open. It is therefore necessary to pro- 
vide auxiliary means for boosting the magneto output 
during the engine starting period, when it is advan- 
tageous to have the spark retarded to some extent. 
Two methods widely adopted are impulse starters and 
booster coils, which arc described in the following para- 
graphs. The retarding of the spark is effected by a 
secondary brush in the distributor arm which "trails" 
the main brush. 


Impulse couplings, or impulse starters as they are 
sometimes called, are used in some small piston 
engine ignition systems and are fitted between the 
magneto shaft and drive shaft. The unit produces a 
heavy spark by giving a magneto armature or magnet 
a brief acceleration at the moment of spark produc- 
tion. In one type of unit the coupling between the 
magneto and engine is a spring-loaded clutch device 
which flicks the armature or magnet through the 
positions at which a spark normally occurs, thus 
momentarily increasing its rotational speed and the 
voltage generated. After the engine is started and the 
magneto reaches a speed at which it furnishes suf- 
ficient output, flyweights in the coupling fly outward 
due to centrifugal force and overcome the springs, so 
that the coupling functions as a solid drive shaft and 
the magneto continues to operate in the normal 


Booster coils, which may be either of the high tension 
impulse or low tension impulse type, derive their 
power from the aircraft's system via either the battery 
or the ground power supply source. The supply is 
controlled either by a separate booster coil or the 

engine starter switch. High tension booster coils 
supply a stream of impulses to the trailing brush of the 
distributor, while in a low tension system a stream of 
impulses is fed to the magneto primary windings either 
to augment or to replace the voltage induced by the 
magnetic flux. In some low tension systems, the 
supply to the primary winding is fed via a second 
contact-breaker, which is retarded in relation to the 
main contact-breaker but connected in parallel with 
it. With this arrangement intermittent high tension 
current is induced in the secondary winding of the 


Ignition systems are controlled by "on-off" switches 
connected in the magneto circuit, but unlike the basic 
and conventional switching arrangements, an ignition 
system switch completes a circuit by closing its con- 
tacts in the "off position. The circuit in this case is 
between the magneto primary winding and ground, 
and since the contact-breaker becomes short- 
circuited, then in the event the magneto is rotated, 
there can be no sudden collapse of the primary win- 
ding flux and therefore no high voltage spark across 
the spark plug gap. 

On dual-ignition systems each magneto may be 
controlled by a separate toggle switch or, as is more 
usual, by a rotary type four-position switch controlling 
both magnetos. The four positions are "off, "left", 
"right" and "both". The left and right positions allow 
one system to be turned off at a time for carrying 
out "magneto-drop" checks during engine ground 

These systems were developed for use on engines 
having a large number of cylinders and designed for 
high altitude operation. They overcome certain 
problems which can occur with high tension systems, 
e.g. breakdown of insulation within a magneto due 
to decreased atmospheric pressure and electrical 
leakage, particularly when ignition harnesses are 
enclosed in metal conduits. Furthermore, the amount 
of cable carrying high voltages is considerably 
reduced. The magneto is similar to a polar inductor 
type of magneto but does not embody a secondary 
coil. Low voltage impulses from the magneto primary 
winding are supplied directly to the distributor, which 
also differs from the types normally employed, in 
that voltage impulses are received and distributed via 
a set of brushes and segmented tracks. The distributor 


output is supplied to transformers corresponding in 
number to the number of spark plugs and located near 
the plugs. Thus, high voltage is present in only short 
lengths of shielded cable. Low tension magnetos are 
switched on and off in the same manner as high ten- 
sion magnetos. 


The function of a spark plug is to conduct the high 

voltage impulses from the magneto and to provide an 

air gap across which the impulses can produce a spark 

discharge to ignite the fuel/air charge within the 


The types of spark plugs used vary in respect to 
heat range, thread size, or other characteristics of the 
installation requirements of different engines, but in 
general they consist of three main components: (i) 
outer shell, (ii) insulator and (iii) centre electrode. 
The outer shell, threaded to fit into the cylinder, is 
usually made of high tensile steel and is often plated 
to prevent corrosion from engine gases and possible 
thread seizure. The threads are of close tolerance and 
together with a copper washer they prevent the very 
high gas pressure escaping from the cylinder. Pressure 
that might escape through the plug is retained by inner 
seals between the outer shell, the insulator, and centre 
electrode assembly. 

The materials used for insulators vary between 
plug designs and applications to specific engines; those 
most commonly adopted are mica, ceramic and 
aluminium oxide ceramic, the latter being specifically 
developed to withstand more exacting mechanical, 
thermal and electrical requirements. Insulation is also 
extended into a screen tube which is fixed to the outer 
shell and provides attachment for the ignition harness 
cable to ensure suppression of radio interference. 

The centre electrode carries the high tension voltage 
from the distributor and is so secured that the requi- 
site spark gap is formed between it and a negative or 
ground electrode secured to the "firing" end of the 
outer shell. Electrodes must operate under very severe 
environmental conditions, and the materials normally 
chosen are nickel, platinum and iridium. 


Almost all piston engines employ two entirely in- 
dependent ignition systems; thus each cylinder has 
two spark plugs, each supplied from a different 
magneto. The purpose of dual ignition is to (i) 
reduce the possibility of engine failure because of an 
engine fault and (ii) reduce the time taken to burn 

the full charge enabling peak gas pressure to be 
reached and thereby increasing engine power output. 
Both magnetos are normally switched by a rotary 
switch in the manner described on page 143. 

The ignition system of a turbine engine is much simp- 
ler than that of a piston engine due to the fact that 
fewer components are required and that electrical 
ignition of the air/fuel mixture is only necessary when 
starting an engine. Another difference is that the 
electrical energy developed by the system is very much 
higher in order to ensure ignition of atomized fuel 
under varying atmospheric and air mass flow condi- 
tions and to meet the problems of relighting an engine 
in the air. 

The principal components of a system are a high- 
energy ignition unit and an igniter plug inter-connected 
as shown in Fig. 1 1.4. Two such systems are normally 
fitted to an engine, the igniter plugs being located in 
diametrically-opposed combustion chambers to ensure 
a positive and balanced light-up during starting. Direct 
current from the aircraft's main busbar is supplied to 
an induction coil or a transistorized high tension 
generator within the ignition unit in conjunction with 
the starter system, and also independently through 
the "relight" circuit. The coil, or generator, as approp- 
riate, repeatedly charges a reservoir capacitor until its 
voltage, usually of the order of 2,000 volts, is suffi- 
cient to break down the sealed discharge gap. The gap 
is formed by two tungsten electrodes within a chamber 
exhausted of air, filled with an inert gas and sealed to 
prevent oxidation which would otherwise occur with 
the large current handled. 

The discharge is conducted through a choke, which 
extends the duration of the discharge, and through a 
high tension lead to the igniter plug (see Fig. 1 1 .5) at 
which the energy is released. A pellet at the "firing" 
end of the plug has a semi-conducting surface, and 
during operation this permits a minute electrical 
leakage from the centre electrode to the body, thereby 
heating the surface. Due to the negative temperature/ 
resistance characteristics of the pellet a low resistance 
path is provided for the energy, which discharges 
across the surface as a high intensity flashover as 
opposed to a spark jumping an air gap. The capacitor 
recharges and the cycle is repeated approximately 
once every second. Once the fuel/air mixture has 
been ignited, the flame spreads rapidly through balance 
pipes which interconnect all the combustion cham- 
bers; thus combustion is self-sustaining and the igni- 


28-V d.c. bjs-bor 

Higl energy ignition unit 


Ignition switch 



Ignition on 

Fig 11.4 

High-energy ignition system 

tion system can be switched off. The energy stored in 
the capacitors is potentially lethal, and to ensure their 
discharge when the d.c. supply is disconnected, the 
output is connected to ground via a safety resistor. 

The electrical energy supplied by the ignition unit 
is measured in joules, and independent ignition systems 
normally consist of two units rated at 12 joules each. 

In the event that through adverse flight conditions 
the flame is extinguished, the engine is "relit" by 
switching on the ignition system until the engine runs 
normally again. During relighting it is unnecessary 
to use the starter motor since the engine continues to 
rotate under the action of "windmilling". In some 


h.t Eiecfoae 



Contact button 

■Locking serrations 

Fig 11.5 

High-energy igniter plug 

cases, relighting is automatic by having one of the 
two ignition units of a low rating (usually 3 joules) 
and keeping it in continuous operation. Where this 
method is not desirable a glow plug is sometimes 
fitted in the combustion chamber where it is heated 
by the combustion process and remains incandescent 
for a sufficient period of time to ensure automatic 

Fire Detection and Extinguishing Systems 

Fire is, of course, one of the most dangerous threats 
to an aircraft and so precautions must be taken to 
reduce the hazard, not only by the proper choice of 
materials and location of equipment in potential 
fire zones, but also by the provision of adequate fire 
detection and extinguishing systems. These systems 
may be broadly classified as (i) fixed, some examples 
of which are used mainly for engine fire protection, 
and detection of smoke in baggage compartments, 
or (ii) portable, for use in the event of cabin fires. 
Both systems are employed in all aircraft except cer- 
tain small low-powered piston cngined types which, 
having been certificated as constituting a negligible 
fire risk, at most need only a portable extinguisher 
within the cockpit. Fixed detection and extinguishing 
systems only, require electrical power for their opera- 
tion, and some typical examples are described in the 
following paragraphs. 



A fire detection system is installed mainly in engine 
compartments, and consists of special detecting 
elements strategically positioned within several fire 
zones designated by the aircraft manufacturer. The 
elements, which may be of the "unit" or "spot" type 
or the "continuous" wire type, arc connected to 
warning lights and/or bells, and either type may be 
used separately, or together in a combined fire 
warning and engine overheat system. 

Unit type detectors are situated at points most 
likely to be affected by fire, e.g. in an engine breather 
outlet pipe, and the one most often used in engine com- 
partments is of the differential expansion switch type, 
the principle of which was described on p. 107. 
These detectors may also be used for sensing an over- 
heat condition in areas of the airframe structure 
adjacent to ducting supplying hot air for air- 
conditioning or de-icing systems. 

In order to provide maximum coverage of an engine 
fire zone and to eliminate the use of a considerable 
number of unit detectors, a continuous wire type 
detector system is normally used. The elements of a 
typical system take the form of various lengths of 
wire embedded in a temperature sensitive material 
within a small bore stainless steel or Inconel tube, 
and joined together by special coupling units to form 

a loop which may be routed round the fire zone as 
required. The wire and tube form centre and outer 
electrodes respectively and are connected to the air- 
craft's power supply via a control unit. The power 
supply requirements are 28 volts d.c. and 115 volts 
a.c. or, in some systems, 28 volts d.c. only. Depending 
on the type of control unit the method of operation 
may be based on either variations in resistance or in 
capacitance with variations in temperature of the ele- 
ment filling material. 

The electrical interconnection of components 
normally comprising a system is shown in Fig. 1 1 .6; the 
control unit in this case is of the type employed with 
a variable resistance system. The a.c. supply is fed to 
a step-down transformer, while d.c. is supplied to the 
warning circuit via the contacts of a warning relay, 
the coil of which may be energized by the rectified 
output from the transformer secondary. With the test 
switch in the normal position, the ends of the centre 
wire electrode of the element are connected in parallel 
to the rectifier and to one end of the transformer 
secondary winding. The other end of the winding is 
connected to the outer tube or electrode so that the 
current path is always through the filling material, the 
resistance of which will govern the strength of recti- 
fied current flowing through the relay coil. With this 
arrangement the warning function is in no way affected 



Fig 11.6 
Fire detection system 


in the event that a break should occur in the loop. 

Under normal ambient temperature conditions the 
resistance of the filling material is such that only a 
small standing current flows through the material; 
therefore, the current flowing through the warning 
relay coil is insufficient to energize it. In the event of 
a temperature rise the resistance of the filling material 
will fall since it has an inverse characteristic, hence 
the rectified current through the relay coil will in- 
crease, and when the fire zone temperature has risen 
to such a value that the relay coil current is at a pre- 
determined level, it will energize the relay thereby 
completing the warning light or bell circuit. When the 
temperature falls and the current drops to a pre- 
determined level the relay de-energizes and the system 
is automatically reset. 

In a capacitance system the detector element is 
similar in construction to that earlier described, but 
in conjunction with a different type of control unit 
it functions as a variable capacitance system, the 
capacity of the element increasing as the ambient 
temperature increases. The element is polarized by the 
application of half-wave rectified a.c. from the control 
unit, which it stores and then discharges as a feedback 
current to the gate of a silicon-controlled rectifier 
(SCR) in the control unit during the non-charging half 
cycles. When the fire zone temperature rises the feed- 
back current rises until at a pre-determined level the 
SCR is triggered to energize a fire warning light, or 
bell, relay. A principal advantage of this sytem is that 
a short circuit grounding the element or system 
wiring does not result in a false fire warning. 

When the test switch is set to the "Test" position, 
the test relay is energized and its contacts change 
over the supply from the rectifier so that the current 
passes directly along the centre electrode. Thus, if 
there is no break in the loop there is minimum resis- 
tance and the warning relay circuit is actuated to 
simulate a fire warning and so indicate continuity. 


In many of the larger types of transport aircraft, the 
freight holds, baggage compartments and equipment 
bays are often fitted with equipment designed for the 
detection of smoke. Detection equipment varies in 
construction, but in most cases the operation is based 
on the principle whereby air is sampled and any 
smoke present, causes a change of electric current 
within the detector circuit to trigger a warning system. 

As an illustration of the operating principle we 
may consider the detector shown in Fig. 11.7, 

which is of a type commonly used. The detecting 
circuit is made up of two photo-electric cells located 
in separate compartments and being affected by the 
light from a single projector lamp. The cells form part 
of an electrical bridge circuit which under normal 
circuit conditions is balanced and so passes no current. 
In the event that smoke is present it passes through 
only one of the detector compartments, thereby in- 
creasing the light scatter and unbalancing the bridge 
circuit. The current resulting from this unbalance 
energizes relays which complete the power supply 
circuit to the appropriate warning system. The power 
supply required for smoke detection systems is nor- 
mally 28 volts d.c. 


Fixed fire extinguishing systems are used mainly for 
the protection of engine installations, auxiliary power 
units, landing gear wheel bays and baggage compart- 
ments, and are designed to dilute the atmosphere of 
the appropriate compartments with an inert agent 
that will not support combustion. Typical extin- 
guishing agents are methyl bromide, bromochlorodi- 
fluoromethane or freon, and these are contained 
within metal cylinders or "bottles" of a specified 
capacity. The agents are pressurized by an inert gas, 
usually dry nitrogen, the pressures varying between 
types of extinguisher, e.g. 250 lbf/in 2 for 12 pounds 
of methyl bromide, 600 lbf/in 2 for 4 pounds of freon. 
Explosive cartridge units which are fired electrically, 
are connected to distributor pipes and spray rings 
located in the potential fire zones. Electrical power 
for cartridge unit operation is 28 volts d.c. and is 
supplied from an essential services busbar; the 
circuits are controlled by switches located in the cock- 
pit and, in some aircraft engine installations, by crash 

When the cartridge unit is fired a diaphragm is 
ruptured and the appropriate extinguishing agent is 
discharged through the distributor pipes and spray 

In the fire extinguisher systems of some types of 
aircraft, electrical indicators are provided to show 
when an extinguisher has been fired. An indicator 
consists of a special type of fuse and holder connected 
in the extinguisher cartridge unit circuit. The fuse 
takes the form of a small match-head type charge 
covered by a red powder and sealed within the fuse 
body by a disc. A transparent cover encloses the top 
of the fuse body and is visible through another cover 
screwed on to the fuse holder. 






Light obsorbmg 


^Sliding cover 

Snapslide Balancing \ 

fastene- shutter \ 


«M Window 




Stutter od|usting 


Resilient mounting 

Fig 11.7 

Smoke detector 

The fuse is secured in the fuse holder by a bayonet 
type fixing, and electrical connection to the charge 
is by way of terminals in the fuse holder, contact at 
the base of the fuse and the metal disc. 

When current flows in an extinguisher cartridge 
circuit, the appropriate fuse charge is fired, thereby 
displacing the disc and interrupting the circuit. At the 
same time red powder is spattered on to the inside of 
the cover thus giving a positive visible indication of 
the firing of the extinguisher cartridge. 

De-icing and Anti-icing Systems 

Icing on aircraft is caused primarily by the presence 
in the atmosphere of supercooled water droplets, i.e. 
droplets at a temperature below that at which water 
normally freezes. In order to freeze, water must lose 
heat to its surroundings, thus when it strikes, say, an 
aircraft wing, an engine air intake or a propeller, there 
is metal to conduct away the latent heat and the 
water freezes instantly. The subsequent build-up of 

ice can change the aerodynamic shape of the particular 
form causing such hazardous situations as decrease 
of lift, changes of trim due to weight changes, loss of 
engine power and damage to turbine engine blading. 
In addition, loss of forward vision can occur due to 
ice forming on windshield panels, and on an excres- 
cence such as a pressure head, obstruction of the 
pressure holes will result in false readings of airspeed 
and altitude. Therefore, for aircraft which are in- 
tended for flight in ice-forming conditions, protective 
systems must be incorporated to ensure their safety 
and that of the occupants. 

There are three methods adopted in the systems 
in common use and these together with their applica- 
tions and fundamental operating principles are set out 
in Table 11.1. They are all based on two techniques, 
known respectively as de-icing and anti-icing. In de- 
icing, ice is allowed to build up to an extent which 
will not seriously affect the aerodynamic shape and 
is then removed by operation of the system; this cycle 
is then continuously repeated, usually by a timing 


Table 11.1 





Wings, tail units, 
propellers, windshields 

Wings, tail units 


A chemical which breaks down the bond 
between ice and water and can be either 
sprayed over the surface, e.g. a windshield, 
or pumped through porous panels along 
the leading edge of a surface, e.g. a wing. 

Sections of rubber boot along the leading 
edges are inflated and deflated causing 
ice to break up and, with aid of the air- 
stream, crack off. 


(a) Hot air bleed 

(b) Combustion heating 

(c) Electrical heating 

Wings, tail units, engine 
air intakes 

Wings, tail units 

Wings, tail units, engine 
air intakes, propellers, 
helicopter rotor blades, 

Hot air from turbojet engine compressors 
passed along inside of leading edge 

Hot air from a separate combustion heater 
or from a heat exchanger associated with 
a turbine engine exhaust gas system. 
Heating effect of electric current passing 
through wire, flat strip or film type 

device. In anti-icing, the system is in operation con- 
tinuously so that ice cannot be allowed to form. 

Electrical power and certain electrical components 
are required in varying degrees for all the systems listed 
in Table 11.1. In fluid, hot air bleed and combustion 
heating systems the requirements are fairly simple 
since it is usually only necessary to operate an electrical 
pump, air control valves and temperature-sensing sys- 
tems as appropriate. The requirements for pneumatic 
boot systems are also fairly simple, although the 
number of air control valves is increased proportion- 
ately to the number of boot sections necessary and an 
electronic timer is used. 

In what may be termed "pure electrical heating 
systems", the application of electrical power and 
components is much wider and as a result the systems 
are of a more complex nature. It is beyond the scope 
of this book to go into the construction and operating 
details of any one specific system, but the following 
details, although of a general nature only, may never- 
theless, be considered as typical. 

A system is made up of three principal sections: 
heating elements, control, protection and indicating. 
The power supplies normally required are 1 15 volts 
to 200 volts a.c. for heating (although the propellers 
for some light aircraft types and some windshield 
panels operate on 28 volts d.c), 115 volts a.c. and 28 
volts d.c. for control and for other sections of a sys- 

tem. Depending on the application, heating current 
may be controlled to permit de-icing, anti-icing or 

The heating elements vary in design and construc- 
tion depending on the application. For propellers they 
are of the fine wire type sandwiched in insulating and 
protective materials which form overshoes selected 
for maximum resistance to environmental conditions 
and bonded to the blade leading edges. For turbine 
engine air intakes, leading edges of wings, tail units 
and helicopter rotor blades, the elements are of the 
"sheared foil" type, i.e. they arc cut from thin sheets 
of high-grade metal to specified lengths and widths 
and within very close tolerances. The final resistance 
values of the elements which are selected from such 
metals as nickel, copper-nickel and nickel-chrome, are 
usually adjusted by chemical etching. The elements 
are also sandwiched between insulating and protective 
layers to form overshoes or mats. 

Fig. 1 1.8 illustrates a typical propeller and air 
intake de-icing system. Electrical power, at 200 volts 
a.c. and variable frequency, is supplied to the propel- 
ler blades and spinner, via brushes and slip rings and a 
cyclic time switch, so that during the de-icing part of 
the cycle, heat is applied to all four blades simul- 
taneously. It is unnecessary to de-ice the whole of each 
blade, as kinetic heating allied to centrifugal force 
normally keeps the outer halves free from ice. 


Propeller blades 


intake leading edge 
and breaker strips 


Inner and outer surfaces 

Fig 11.8 
Propeller and air intake de-icing system 

The air intake elements are arranged so that those 
positioned at the leading edges are continuously heated, 
i.e. they perform an anti-icing function, while those 
on the inner and outer surfaces are supplied via the 
cyclic time switch and so perform a de-icing function. 
In order that ice may be shed in reasonably-sized 
sections, the leading edge heating elements are ex- 
tended at intervals to form "breaker" strips. The 
resistance of the elements is graded to provide for 
various heating intensities required at different parts 
of the air intake. 

Two other forms of metal element designed for 
the protection of aircraft tail units, air intakes and 
areas where a complex shape or heating pattern is 
required, are the printed circuit element and sprayed 
element. A printed circuit element is manufactured 
from a thin sheet of metal foil (usually pure nickel) 
coated with epoxy resin on one side; the element is 
then printed on the other side of the foil in an acid- 
resistant ink. The foil is placed in an acid bath and the 
"uninked" parts are etched away leaving the element 
cut to dimensions. The ink is then removed from the 
element, and a final cleaning and immersion treatment 
is carried out to obtain the required resistance; the 
epoxy resin is removed before final assembly to the 
heater overshoe or mat. 

The second form of element is one which is 
applied direct to the component to be protected. The 
component is lightly abraded to give a good adhesion 
surface and is then sprayed in thin layers with hot- 
setting araldite. Each layer is cured individually, and 

after building up to a specified thickness, the heating 
element is "masked out" on the surface and aluminium 
is applied as a sprayed metal process. The resistance is 
accurately measured during the process and is adjusted 
by rubbing down or adding metal, and the clement is 
finally finished by applying further layers of araldite. 

For windshields or other essential clear vision panels 
in cockpits, a transparent metal film type of element 
is employed, the metal being either stannic oxide or 
gold. Panels are of laminated construction, and in order 
to provide rapid heat transfer the metal film is elec- 
trically deposited on the inside of the outer glass layer. 
It is protected from damage and completely insulated 
by further layers of polyvinyl butyral, glass and/or 
acrylic. Heating current, normally from an a.c. source, 
is supplied to the film by metal busbars at opposite 
edges of the glass layer. The power necessary to deal 
with the most severe icing conditions is in the order 
of 5—6 watts/in 2 of windshield area. 

Windshield systems are essentially anti-icing sys- 
tems for, in addition to the protective function, the 
temperature of the panels must be higher than ambient 
during take-off, flight at low altitudes and landing, 
thus making them "pliable" and thereby improving 
their impact strength against possible collision with 

In view of the high amounts of power required for 
the foregoing electrical heating methods, it is essential 
to provide each system with appropriate controlling 
circuits and devices. Although there are a number of 
variations between systems and between designs 
adopted by different manufacturers, from the point 
of view of primary functions they are more or less 
the same, i.e. to cycle the power automatically, to 
detect any overloading and to isolate power supplies 
under specific conditions. We may therefore consider 
Fig. 1 1.9, which is based on the engine and propeller 
system earlier described, as being a typical circuit. 
When the system is switched on, direct current 
energizes the power relay via closed contacts in the 
overload sensing device, thus allowing the 200 volts 
a.c. to flow directly through to the continuously 
heated elements and up to the time switch (see also 
p. 104). This unit is energized to run either "fast" or 
"slow" by a selector switch, the settings being 
governed by outside air temperature and severity of 
icing. In this case, "fast" is selected at temperatures 
between +10°C and -6°C and the duration of the 
"heat on" and "heat off periods of the cyclic heated 



Landing gear 
' microswitch 

To reduced 

voltage section of oiternator 

control unit 


Air intake elements 

Propeller elements 

Fig 11.9 
Electrical de-icing and anti-icing system 

elements is short compared with "slow", which is 
selected at temperatures below -6°C. The cycling is 
usually controlled by cam-operated mtcroswitches. 
An indication of time switch operation is provided by 
a flashing blue or green light on the control panel, 
while a general indication that the correct power is 
being applied to the whole system is provided by an 
ammeter connected to a current transformer (see 
also p. 42) across the generator busbar. 

In the event of an a.c. overload, the heater elements 
are protected by the sensing device which is actuated 
in such a manner that it interrupts the d.c. supply to 
the power relay, this in turn interrupting the supply 
of heating current. The current balance relay fulfils 
a similar function and is actuated whenever there is 
an unbalance between phases beyond a predetermined 

For operation on the ground, it is usual for the 
applied voltage to be reduced in order to prevent over- 
heating. This is effected by the automatic closing of a 
microswitch fitted to a landing gear shock-strut, the 
switch permitting direct current to flow to a reduced 
voltage control section within the generator voltage 

The control methods adopted for windshield anti- 
icing systems are normally thermostatic, and a typical 
system (Fig. 1 1.10) consists of a temperature-sensing 
element and a control unit. The element is embedded 
Within the panel in such a way that it is electrically 
insulated from the main heating film and yet is 
capable of responding to its temperature changes 
without any serious lag. A control unit comprises 
mainly a bridge circuit, of which the sensing element 
forms part, an amplifier and a relay. When all the 
required power is switched on initially, the control 
unit relay is energized by an unbalanced bridge signal 
and the power control relay is energized to supply the 
windshield panel. As the panel temperature begins to 
increase, the sensing element resistance also increases 
until at a predetermined controlling temperature (a 
typical value is 40°C) the current flowing through the 
sensing element balances the bridge circuit, and the 
control unit and power control relays are de-energized, 
thereby interrupting the heating current supply. As 
the temperature cools the sensing element resistance 
decreases so as to unbalance the bridge circuit and 
thereby restore the heating current supply. In a num- 
ber of aircraft types the windshields are each fitted 


200-Vo.c. heoting 

— Control — 
28-Vd.c. 115-Vo.c. 

Windshield ponel 

Fig 11.10 
Schematic arrangement of windshield anti-icing control system 

with an additional overheat sensing element which in 
the event of failure of the normal sensing element 
takes over its function and controls at a suitably higher 
temperature; 55°Cis a typical value. 

Despite accurate control during manufacture slight 
variations in heater film resistance, and consequently 
glass temperature, can occur. Sensing elements are, 
therefore, individually embedded in each panel at one 
of the hotter spots but where it least affects visibility. 


Ice detectors consist mainly of a probe located at a 
strategic point on an aircraft (usually the front fuse- 
lage section) and a warning light, their purpose being 
to give adequate warning, and an assessment of the 
likely severity of an impending icing hazard in suffi- 
cient time for the ice protection systems to be brought 
into operation. Detectors are made in a variety of 
forms, and in those most commonly used actuation of 
the warning circuit is triggered off by ice accretion at 
the probe. In one type of system ice accretion causes 
a drop in pressure sensed by the probe and a dia- 
phragm, the deflections of which make a circuit to 

the warning light and to a heater within the probe. 
When the ice has melted the warning light and heater 
circuits are interrupted and the system is reset for 
further ice detection. A second type of system is 
designed to give a warning and also automatically 
switch on airframe and engine de-icing systems. It 
consists of an a.c. motor-driven rotor which rotates 
in close proximity to a knife-edge cutter, a time delay 
unit and a warning lamp. Under icing conditions ice 
builds up on the rotor and closes the gap between it 
and the cutter. This results in a substantial increase in 
the torque-loading on the detector motor, causing it 
to rotate slightly in its mounting and to trip a micro- 
switch inside the detector. Tripping of the micro- 
switch completes the circuit to the warning lamp and 
time delay unit which initiates operation of the de- 
icing systems. These conditions are maintained until 
the icing diminishes to the point whereby the knife- 
edge cutter ceases to "shave" ice, and the microswitch 
is returned to the open circuit condition. The detector 
unit is designed to provide a two minute interval be- 
tween the cessation of an ice warning and shut down of 
a de-icing system, to prevent continuous interruption 


of the system during intermittent icing conditions. 

In a third type of system the probe is vibrated 
ultrasonically by an oscillator circuit at a resonant 
frequency of approximately 40 kHz. When ice forms 
on the probe the frequency decreases and this is 
detected by comparing it with the frequency from a 
reference oscillator. At a pre-determined frequency 
change a time delay and switch circuit is actuated and 
a warning light comes on. At the same time a heater 
within the probe is switched on to remove ice from the 
probe thereby increasing its frequency again. After 
approximately 60 seconds time delay, the warning 
light is extinguished and the system returns to the 
detector mode of operation, to repeat the cycle 
while icing conditions exist. 

Passenger Cabin Services 

In passenger transport aircraft electrical power is 
required within the main cabin compartments for the 
service and convenience of the passengers, the extent 
of power utilization being governed of course, by the 
aircraft size and number of passengers it is designed to 
carry. Apart from the main cabin lighting referred to 
in Chapter 10 it is necessary to provide such additional 
services as individual reading lights at each seat posi- 
tion, a cabin attendant call system, public address 
system and a galley for the preparation and serving 
of anything from light refreshments to several full- 
course meals. In-flight cinema entertainment also 
accounts for the utilization of electrical power in 
many types of aircraft. 

Reading lights may be of the incandescent or 
fluorescent type, and are located on passenger 
service panels on the underside of hat racks, or in each 
seat headrest and are controlled individually. Cabin 
attendant call systems are interlinked systems com- 
prising switches at each passenger service panel con- 
nected to an electrical chime and indicator light at 
the cabin attendant's panel station. The service panel 
switches are of the illuminating type to visually indi- 
cate to the cabin attendant the seat location from 
which a call has been made. In addition the system 
provides an interconnection between the flight crew 
compartment and cabin attendant's station. 

A public address system is provided for giving 
passengers instructions and route information, and 
usually comprises a central amplifier unit and a num- 
ber of loudspeakers concealed at various points 
throughout the cabin, and in toilet compartments. 
Information is given, as appropriate, by the aircraft's 

captain or cabin attendant by means of separate 
telephone type handsets connected to the loud- 
speakers. Tape-recorded music may also be relayed 
through the system during passenger embarkation and 

Galley equipment has a considerable technical 
influence on the design of an aircraft's electrical 
system, in that it represents a very high percentage 
of the total system power requirement, and once 
installed it usually becomes a hard-worked section of 
an aircraft. The type of equipment and power loadings 
are governed by such factors as route distances to be 
flown, number of passengers to be carried and the 
class configurations, i.e. "economy", "first-class" or 
"mixed". For aircraft in the "jumbo" and "wide- 
bodied" categories, galley requirements are, as may 
be imagined, fairly extensive. In the Boeing 747 for 
example, three galley complexes are installed in the 
cabin utilizing both 28 volts d.c. and 115 volts a.c. 
power and having a total power output of 140 kVA; 
thus, assuming that the generator output is rated with 
a power factor of unity, the equivalent d.c. output is 
140 kilowatts or in terms of horsepower approxi- 
mately 187! The galle> unit of the wide-bodied Lock- 
heed "Tristar" is also a complex unit but is located as 
a central underfloor unit. It also utilizes d.c. and a.c. 
power not only for heating purposes but also for the 
operation of lifts which transport service trolleys to 
cabin floor level. 

The equipment varies, some typical units being 
containers and hot cups for heating of beverages, hot 
cupboards for the heating of pre-cooked meals and ovens 
for heating of cold pre-cooked meals, a number of 
which may have to be served, e.g. on long-distance 
flights. Other appliances required are water heaters 
for galley washing-up and toilet washbasins, and re- 
frigerators. In most cases, the equipment is assembled 
as a self-contained galley unit which can be "plugged 
in" at the desired location within the aircraft. 

It is usual for the electrical power to be supplied 
from the main distribution systems, via a subsidiary 
busbar and protection system, and also for certain 
galley equipment to be off-loaded in the event of 
failure of a generating system. The load-shedding 
circuit is automatic in operation and any override 
system provided is under the pilot's control; on some 
aircraft load-shedding is also controlled via a landing 
gear shock-strut microswitch thereby conserving 
electrical power on the ground. The control panel or 
panels, which may be mounted on or adjacent to the 
galley unit, incorporates the control switches, indi- 


System Guidance 




Autopilots & 
Flight Guidance 


Doppler Inertial Navigation Communication 

Fig. 11.11 

Typical applications of avionics 

Engine Power Integrated 
& Flight 

Systems Control Systems 

cator lights and circuit breakers associated with each 
item of galley equipment, and also the indicator 
lights of the cabin attendant call system. 


The term "avionics" is one which defines the applica- 
tion of electronic principles to equipment used in 
aeronautics; in fact the term may be considered as a 
contraction of aviation electronics. In its turn, 
"electronics" may be considered as a designation of 
equipment incorporating either electronic tubes, 
transductors, transistors, silicon diodes and perfor- 
ming such functions as signal amplification, phase 
discrimination. and computing of in-flight data. 

The application of avionics covers a very wide 
area each part fulfilling a specialized function. 
Descriptions of these functions are beyond the scope 
of this Chapter (indeed of the whole book!) but some 
indication of typical applications is given in Fig. 1 1.1 1. 
The extent to which the associated equipment and 
electrical power are required is governed by the type 
and size of aircraft. 

Test Questions 

1 . By means of a diagram show the interconnection 
of the components of a simple engine starting 

2. In terms of cranking speeds what are the differences 
between starter motor requirements for recipro- 
cating and turbine engines? 

3. The self-sustaining speed is the: 

(a) maximum speed at which the starter motor 
runs to maintain rotation of an engine. 

(b) speed at which the engine is capable of main- 
taining rotation. 

(c) speed at which current to the motor is inter- 

4. What type of motor is used for engine starting 

5. What is the function of an overspeed relay fitted 
in some turbine engine starting systems? Describe 
how it fulfils this function. 

6. The purpose of a "blow-out" cycle is to: 

(a) remove excess air from an engine during 

(b) blow cooling air through the starter motor 
after starting. 

(c) remove the unburnt fuel from an engine in the 
event of an unsuccessful start. 

7. In systems incorporating a "blow-out" facility, 
why is it necessary for the motor running time to 
be limited? 

8. Describe the operation of a typical starter-generator 

9. The contact breaker of a magneto is connected in 

(a) primary winding circuit. 

(b) secondary winding circuit. 

(c) circuit between distributor and spark plugs. 

10. Explain how the rate of collapsing of the primary 
winding flux is increased. 

11. What is the formula for calculating the speed of a 

12. A rotating armature magneto to be fitted to a 6- 
cylindcr engine must be driven at: 

(a) the same speed as the engine. 

(b) half the speed of the engine. 

(c) one and a half times the engine speed. 

13. Why is it necessary for the output of a magneto to 
be boosted during starting? Describe a method of 
achieving this. 

14. In what manner does an ignition switch differ from 
a conventional type of switch? 

15. For what purpose were low tension magneto sys- 
tems introduced and what are the essential dif- 
ferences between them and high tension systems? 


16. What are the materials generally used for the tern, and include a suitable test circuit State any 

insulators and electrodes of spark plugs? advantages and disadvantages of the system 

17 What are the essential differences between a turbine described. rvTl ' 'La 

enrine ignition system and the system used for a 21 . Describe the operation of an electrically-operated 
reciprocating engine? fire extinguisher. ,_..,„ 

18. With the aid of a circuit diagram, explain the 22. Describe the two techniques de-icing and anti- 
operation of a high energy ignition unit, icing". O ■•••'' 

(S.L.A.E.T.) 23. What types of heating elements are employed in 

19. What is the purpose of a "relight" circuit and what the various electrical de-icing and anti-icing sys- 
methods are adopted? tems? , , . ... 

20 With the aid of a circuit diagram, describe the 24. Describe the operation of a control method 
' operation of an aircraft engine fire detection sys- adopted in a typical windshield ant.-icing system. 


A.C. exciter, 33, 36 
A.C. generators - 

constant frequency, 32, 36 
frequency-wild, 31, 35 
A.C. motors, 126 
A.C. power supplies - 

compounding transformer, 36 

constant-frequency, 32, 40, 42 

constant-speed drive, 32 

field excitation, 35 

frequency-wild, 31,41 

kilovolt-amperes reactive (KVAR), 

load controller, 42 

load sharing, 4 1 

Merz-Price system, 31 

mutual reactor, 44 

voltage regulation, 38, 40 
A.C. principles - 

active component, 30 

amplitude value, 27 

apparent power, 30 

cycle, 27 

'delta' connection, 30 

effective power, 30 

effective value, 28 

frequency, 27 

in-phase, 28 

instantaneous value, 27 

interconnection of phases, 29 

kilovolt-amperes, 30 

line voltage, 29, 52 

neutral point, 29 

out-of phase, 28 

peak value, 27 

phase angle, 28 

phase relationships, 28 

phase voltage, 29 

phasing, 28 

power factor, 30 

quadrature, 28 

quadrature component, 30 
reactive component, 30, 43 
root mean square value, 27 
sine wave, 27 
single-phase, 28 
'star' connection, 29 
three-phase, 28 
true power, 30 
volt-amperes reactive, 77 
wattful component, 30 
wattless component, 30 
working component, 30 

Advisory lights, 78 

Air-driven generators, 45 

Air turbine, 45 

Aluminium cable connections, 90 

Ammeters, 71 

Amortisseur windings, 34 

Annunciator panel, 79 

Anti-collision lights, 129 

Anti-icing systems, 148 

Apparent power, 30 

Armature, 6 

Armature reaction, 4 

A.T.A. Specification 100, 96 

Auto-transformer, 55 

Auxiliary interpoles, 4 

Auxiliary power units, 67 

Avionics, 155 

Baretter, 38 
Base, 39 
Batteries - 

capacity, 20 

chemical reactions, 17, 18, 20 

connections, 22 

discharge rate, 20 

functions, 16 

lead-acid, 17 

location, 21 

nickel-cadmium, 18, 19 

state of charge, 20 

thermal runaway, 21 

ventilation, 22 
Battery charging unit, 25 
Battery systems, 22 
'Blow out' position, 140 
Bonding system, 93 
Booster coils, 143 
Bridge lighting, 134 
'Bridge' rectifier connections, 32, 

35,37,40, 51 
Brushes, 6, 7 
Brushless generators, 33 
Brush wear, 7 
Busbars, 81 

Cables - 

co-axial, 88 

coding schemes, 84, 97 

connections, 89 

ignition, 87 

routing, 86 

seals, 86 

terminations, 90 

thermocouple, 87 

types, 84 
Cable seals, 86 
Capacitive circuit, 28 
Capacitive load, 56 
Carbon pile regulator, I 1 , 60 
Caution lights, 77 
Central warning systems, 79 
Circuit breakers, 1 13 
Circuit controlling devices — 

relays, 107 

switches, 99 
Circuit diagrams, 96 
Circuit protection devices, 1 1 1 
Circular-scale indicator, 71 
Coaxial cables, 88 
Cockpit lighting, 132 


Coding schemes, 84, 97 
Collector, 39 
Commutator, 1 
Compensating windings, 4 
Compound motor, 122, 139 

Conduits, 86 

Constant-frequency generators, 36 

Constant-frequency systems, 32, 40, 

Contactors, 1 16 
Corona discharge, 95 
Crimped terminals, 90 
Current limiters, 1 12 
Current transformer, 55 

Damper windings, 34 
D.C. generators 

characteristics, 3 

classifications, 2 

cooling, 8 

couplings, 8 

load-sharing, 13 

principles, 1 

shunt-wound, 3 

spark suppression, 7 

voltage regulations, 9 

windings, 4 
D.C. motors, 120 
'Dead beat' indications, 71 
De-icing systems, 148 
'Delta' connection, 30, 54 
Dimming facility, 78 
'Drop-out' voltage, 107 
Dual ignition, 144 
Ducted loom, 86 

Earthing, 89 

Earth stations, 89 

Earth return system, 89 

Eddy current damping, 71 

Effective power, 30 

Electrical bonding, 93 

Electroluminescent lighting, 135 

Emergency lighting, 136 

Emitter, 39 

Equalizing coils, 13 

Essential services, 82 

External characteristics (generator), 

External lighting, 128 

Field excitation 

a.c. generators, 35 

d.c. generators, 3, 9 
Field 'flashing', 4 

Fire detection, 145 

Fire extinguishing, 147 

Frequency, 27 

Frequency meters, 76 

Frequency regulation (inverters), 60 

Frequency-wild generators, 31, 35, 

Frequency-wild systems, 31,41 
Full-wave rectification, 5 1 
Fuses, 1 1 1 

Galley equipment, 153 
'Ganging', 100 
'Gate', 49 
Generators — 

a.c, 31 

air-driven, 45 

d.c, 1 
Generator cooling, 8 
Ground power supplies - 

a.c. systems, 66 

auxiliary power units, 67 

d.c. systems, 65 

Half-wave rectification, 51 
Heat sinks, 32 
'Holes', 39, 47 
Hysteresis motor, 1 28 

Ice detector, 152 
Ignition systems — 

cables, 87 

distributor, 142 

dual, 144 

high energy, 144 

magneto, 141 

spark plugs, 144 

switches, 143 
Indicating fuse, 147 
Indicating lights, 77 
Induction motors, 126 
Inductive circuit, 28 
Inductive load, 56 
In-line connectors, 90 
In-phase, 28 

Instrument lighting, 134 
Internal characteristics (generator), 

Internal lighting, 128, 132, 153 
Intcrpole windings, 4 
Inverter, 59, 60 

'Jumpers', 93 

Kilovolt-amperes (KVA), 30 
Kilovolt-amperes reactive (KVAR), 

Lead-acid battery, 17 
Lighting - 

anti-collision, 129 

cockpit, 132 

emergency, 136 

ice inspection, 1 32 

instrument 134 

landing lamps, 130 

navigation, 128 

passenger cabin, 1 36 

strobe, 130 

taxi lamps, 130 
Limiting resistors, 1 1 2 
Limit switches, 125 
Linear actuators, 124 
Line voltage, 29, 52 
Load controller, 42 
Load equalizing circuit, 1 3 
Load-sharing — 

a.c. generators, 41 

d.c generators, 13 

reactive load, 43 

real load, 42 
Load-shedding circuit, 1 53 
Low tension magneto, 143 

Magnetic amplifiers, 36, 38, 40 
Magnetic indicators, 78 
Magnetic neutral axis, 4 
Magnetic screen, 7 1 
Magnetos, 141 
Measuring instruments — 
ammeters, 71 
frequency meters, 76 
moving coil, 71 
power meters, 77 
shunts, 74 
varmeter, 77 
voltmeters, 7 1 
wattmeter, 77 
watt/var meter, 77 
Mercury switches, 1 04 
Mer/-Price system, 31 
Micro-switches, 103 
Motors - 

actuator, 124 
capacitor, 1 27 
characteristics, 1 2 1 
compound, 122, 123, 139 
hysteresis, 128 
induction, 126 
instrument, 125 
series, 121 
shunt, 122 
split-field, 122 
starter, 122 



Motor-generator, 59 
Moving coil, 71 
Mutual reactors, 44 

Navigation lights, 128 
Neutral point, 29 
Nickel-cadmium battery, 19 
Non-essential services, 82 
'Notch time', 62 
N-P-N transistor, 39 
N-type semiconductor, 39, 47 

Open loom, 86 
Out-of-phase, 28 
Overfrequency protection, 119 
Overvoltage protection, 116 

Paralleling — 

a.c. generators, 41 

d.c. generators, 13 
Passenger cabin services, 136, 153 
'Pigtails', 7 
Pillar lighting, 134 
Plugs, 91 

P-N-P transistor, 39 
Polar inductor magneto, 143 
Polarized armature relay, 109 
'Potting', 92 
Power conversion equipment — 

rotary, 59 

static, 47 
Power distribution systems — 

busbars, 81 

cables, 83 

coding schemes, 84, 97 

earthing, 89 

electrical bonding, 93 

electrical diagrams, 95 

services, 82 

wires, 83 
Power factor, 30 
Power meters, 77 
Power utilization - 

a.c. motors, 126 

actuators, 124 

avionics, 1 54 

d.c. motors, 120 

de-icing, 148 

external lighting, 128 

engine starting, 138 

fire detection, 145 

fire extinguishing, 147 

ignition, 141 

internal lighting, 128, 132 

passenger cabin, 136, 153 
'Press-to-test', 78 

Primary bonding conductors, 93 
Proximity switches, 107 
P-type semiconductor, 39, 47 
'Pull-in' voltage, 107 
Pulse shaper circuit, 62 

Quadrature, 28 
Quadrature component, 30 
Quill-drive, 8 

Reactance sparking, 4 
Reactive component, 30, 43 
Reactive load, 42 
Real load, 42 
Rectifiers — 

circuit connections, 5 1 

'holes', 39, 47 

rotating, 34 

selenium, 48 

silicon, 49 

silicon-controlled, 49 

silicon junction diode, 49 

Zener diode, 40, 49 
Relays - 

attracted-armature, 108 

attracted-core, 108 

overvoltage, 116 

polarized armature, 109, 118 

slugged, 1 1 
'Relight' circuit, 144 
Residual magnetism, 3 
Resistive circuit, 28 
Reverse current circuit breaker, 116 
Reverse current cut-out, 1 15 
Reverse current protection, 115 
Reversible compound motor, 123 
Ripple frequency, 51 
Root mean square value, 28 
Rotary actuators, 124 
Rotary converter, 59 
Rotary converting equipment - 

converter, 59 

frequency regulation, 60 

inverter, 59, 60 

motor-generator, 59 

static inverter, 62 

voltage regulation, 60 
Rotating armature magneto, 142 
Rotating magnet magneto, 143 
Rotating rectifier, 34 
Routing charts, 96 

Screening, 7, 95 

Secondary bonding conductors, 93 
Self-sustaining speed, 139 
Semiconductor, 47 

Series-wound motor, 121 

Shunts, 74 

Shunt motors, 122 

Silicon-controlled rectifier (S.C.R.), 49 

Silicon junction diode, 49 

Single-phase system, 28 

Single-phase transformer, 54 

Slip speed, 127 

Slugged relay, 1 10 

Smoke detectors, 147 

Sockets, 91 

Spark suppression, 7 

Speed characteristic (motors), 121 

Split busbar system, 8 1 

Split-field motor, 122 

Squirrel-cage, 126 

'Star' connection, 29, 54 

Starter-generator, 141 

Starter motor systems, 138 

State of charge, 20 

Static charger, 93 

Static converting equipment — 

rectifiers, 47 

static inverter, 62 

transformers, 53 

transformer-rectifier units, 57 
Static discharge wicks, 95 
Static inverter, 62 
Step-down transformer, 53 
Step-up transformer, 53 
Strobe lighting, 130 
Switches - 

double-pole, 100 

ignition, 143 

limit, 125 

mercury, 104 

micro, 103 

position, 100 

pressure, 104 

proximity, 107 

push, 101 

rheostats, 103 

rocker-button, 102 

rotary, 102 

selector, 99 

single-pole, 100 

thermal, 106, 146 

time, 103, 150 

toggle, 100 
Synchronizing lights, 78 
Synchronous speed, 127 

Temperature control (de-icing and 

anti-icing), 150 
Thermal runaway, 21 
Thermistor, 37 


Thermocouple cables, 87 Transformer-rectifier units (T.R.U's), Voltage drop, 3 

Three-phase system, 28 57 Voltage regulation - 

Three-phase transformer, 54 Trans-illuminated panel, 135 constant-frequency generators, 40 

Thyristor, 49 Transistors, 39 d.c. generators, 9 

Torque characteristics (motor), 121 Transistorized voltage regulator, 39 frequency-wild generators, 38 

Transformation ratio, 53 'Trip-free' circuit breaker, 113 inverters, 60 

Transformers — Turbine engine ignition, 144 Voltage transformer, 53 

auto, 55 Turbine engine starting, 138 Voltmeters, 7 1 

circuit connections, 54 Turbo-starter systems, 140 «/„„,„„ i;„htc n 

compounding, 36 Turns ratio, 53 EESStt' 

current, 42, 55 Wattmeter, 77 

instrument, 74 Underfrequency protection, 1 19 Watt/var meter, 77 

ratings, 56 Undervoltage protection, 1 18 w ' re s, 83 

single-phase, 54 Winn 8 diagrams, 96 

step-down, 53 Varmeter, 79 Yoke, 4 

step-up, 53 Vibrating contact regulator, 9 

three-phase, 54 Vital services, 82 Zener diode, 40, 49 

voltage, 53 Volt-amperes reactive, 77 Zener voltage, 49 

Written by a professional engineer, this is a high-level introductory survey 
of aircraft electronics, providing information on the operating principles 
of the systems and equipment used in aircraft for the generation, control, 
distribution and utilization of electrical power. The systems and equipment 
described are representative of those installed in a wide range of aircraft 
types currently in service. 

An invaluable text for flight and ground engineers, aircraft manufacturers 
and users (including commercial pilots, and students having obtained 
or seeking to obtain basic qualifications).